How to read a WebPageTest Waterfall View chart

Table of contents

I often find myself looking at WebPageTest (WPT) waterfall charts, but as I seem to have the memory of a chimpanzee (not a goldfish, that’s a myth), I tend to forget some of the details and what they all mean. So I decided to pull together many bits of information into a single blog post I can refer to at a later date. If you find it useful, or think I’ve forgotten anything, please let me know. If you are interested in learning about the Connection View that sits directly below the Waterfall View, then check out my post ‘How to read a WebPageTest Connection View chart’.

Basic Layout

Here’s the basic layout of the waterfall chart user interface:

Basic layout of the Web Page Test waterfall chart includes the 1:key, 2:request list, 3:request timeline, 4:CPU utilisation, 5:Bandwidth in, 6:browser main thread, 7:page is interactive.

1 - Key

The key shows three types of information:

  1. Information about the connection status (DNS lookup, connection established, SSL negotiation)
  2. The type of resource being requested (e.g. HTML, images etc)
  3. Miscellaneous events (wait, JavaScript execution)

Each resource has 2 colours, a light shade and a dark shade. The light shade signifies the point at which the browser has made the request for the resource. The dark shade is the point at which the resource is actually downloading. See this comment from Pat Meenan for more information.

The “wait” visualisation is a new addition to WPT. It shows you the time between when the browser first discovered the resource on the page, up to when the request for the resource was made by the browser to the server.

2 - Request list

A list of assets the browser has discovered on the page and the order in which they were requested. Note the request number on the far left, as well as the yellow lock if the request is being made over a secure connection (HTTPS).

3 - Request timeline

The timeline shows the time along the horizontal (x) axis verses each request made on the vertical (y) axis. From this you can see the lifecycle of a request made by the browser. From discovery (wait), through to request being made, and finally to asset being download.

Ideally you want to make sure this timeline covers as little time as possible, as this indicates better overall performance. The thinner the timeline, the quicker the page loads for a user.

4 - CPU Utilisation

A simple graph showing the CPU utilisation of the browser process running on the device. It displays how much CPU the current webpage is using at any point in time. It ranges from 0 - 100% utilisation. See this comment from Pat Meenan for more information.

5 - Bandwidth In

This is an indicator of when data is coming into the browser. The visualisation is helpful to see when the browser is doing useful work vs wasted time. The absolute scale can be ignored, as it isn’t very accurate. Use the “Capture network packet trace (tcpdump)” option in the advanced tab on the WebPageTest homepage if you want more accurate results. See this comment from Pat Meenan for more information.

6 - Browser Main Thread

This graph visualises what the browsers main thread is doing at any specific point in time (x-axis). The y-axis shows the percentage from 0 - 100%. The colours were copied from the Chrome DevTools CPU graph (under the performance tab). Here’s what each of the colours mean:

  • Orange - Script parsing, evaluation and execution  
  • Purple - Layout  
  • Green - Painting  
  • Blue - HTML parsing  

Using this graph it is possible to see if the CPU is becoming a bottleneck in one of the above areas.

7 - Page is Interactive

This graph gives you an indication of when the main thread is blocked. The red blocks indicate that the main thread has been blocked for 100ms (which will also block inputs like button presses). Green indicates the thread isn’t blocked. Note: it still may be possible to scroll during red blocked phases, as scrolling is usually handled off the main thread for most browsers. See this comment from Pat Meenan for more information.

Vertical lines

You can see the key for each of the coloured vertical lines below the “Waterfall View” header as seen below:

Key for the vertical lines displayed on the timeline. From left to right: Start render, RUM first paint, DOM interactive, DOM content loaded, On load, Document complete.

But what do each of them mean?

Start Render - Green line  

This is the first point at which a user will see pixels painted to the page. The pixels could be from anything at all (background colours, borders etc), not necessarily content. Before this point the screen was blank. This metric is measured by analysing individual video frames that are captured during the page load. See comment from Pat Meenan for more information.

RUM First Paint - Light green line  

This is the point where the browser renders anything to the screen that is visually different from before navigation (i.e. the blank screen for WPT). This metric is reported via the browsers API, so when it thinks it painted the first content. Because of this, the line is only visible if the browser supports the Paint Timing API.

DOM Interactive - Yellow line  

The point at which the browser has finished parsing all the HTML, and DOM construction is complete. Unfortunately it’s not a reliable metric.

DOM Content Loaded - Pink line  

The point at which the HTML has been loaded and parsed, and the browser has reached the end of the document. All blocking scripts have been loaded and run. The DOM at this point is completely defined. See comment from Pat Meenan for more information.

On Load - Lavender line  

The point at which the window load event fires. All objects are in the DOM and all images and scripts have finished loading.

Document Complete - Blue line  

The point where the onload event has fired and all the static image content has loaded. Content changes that are triggered by JavaScript execution may not be included.

Horizontal timings

So let’s now concentrate on the request timeline (3). What do the horizontal blocks mean and what metrics do they refer too? Well, if you click on an individual request you will see a popup with a lot more information, as seen in the example below:

By clicking on an individual request in WebPageTest it will give you much more information including timing details, request details, response details and if enabled when testing, the original response body.

So lets take a look at a few of the requests from this waterfall view, as it gives us a number of quite varied requests to look at.

Request 1 - The HTML

Here the browser is requesting the HTML document, so at this point in time it is also having to setup the connection to the server. In the request details we are given the following timings:

  • Discovered: 0.011 s
  • Request Start: 0.116 s
  • DNS Lookup: 27 ms
  • Initial Connection: 25 ms
  • SSL Negotiation: 43 ms
  • Time to First Byte: 315 ms
  • Content Download: 40 ms

I’ve annotated the request to show what each of these timings mean:

Request number one with timings annotations added, including all the timings listed above.

Adding the DNS, Initial Connection, SSL negotiation, Time to First Byte (TTFB) and the Content download times gives you the 450ms that is displayed directly after the request finishes.

It’s worth noting that WPT follows a specific convention in the request details panel:

  • If the time corresponds to a duration, it is measured is milliseconds (ms), e.g. the DNS lookup took 27ms.
  • If the time corresponds to a starting point, it is measured in seconds (s), e.g. the request started at 0.116s.

Request 7 - A third-party JavaScript file

This request is different from the other requests examined because the file is coming from a different domain. The request details give the following timings:

  • Discovered: 0.473 s
  • Request Start: 0.702 s
  • DNS Lookup: 28 ms
  • Initial Connection: 39 ms
  • SSL Negotiation: 153 ms
  • Time to First Byte: 48 ms
  • Content Download: 9 ms

Notice how the browser needs to go through the whole connection negotiation again (DNS, Connection, SSL negotiation) because the file exists on a different domain. This adds a fair chunk of time to the request (28 + 39 + 153 = 220ms).

Request 7 loads a JavaScript file from a third-party domain. This causes a new TCP connection to be established. 200ms after the JavaScript is downloaded the graph shows JavaScript execution that is also visible on the Browser main thread.

The other interesting point about this request is the script executes around 200ms after the download is complete. There’s no information about this execution in the details panel, but you can see it in the waterfall as light pink lines after the request and orange in the “Browser Main Thread” panel (6) which signifies “Script parsing, evaluation and execution”.

Request 15 - A sponsor PNG

With this request the browser has discovered a PNG image and requests it from the server. In the request details we are given the following timings:

  • Discovered: 0.652 s
  • Request Start: 0.824 s
  • Time to First Byte: 214 ms
  • Content Download: 28 ms

Wait time is calculated by subtracting the discovered time from the request start time. The wait time is the time taken from when the browser first finds the asset, to the time when it has the capacity to send a request to the server.

Request 15 downloads a PNG image. The waterfall now includes the new waiting period from when the asset was discovered by the page, to when the browser makes the request.

The duration after this request is the time taken from the request being made, to when the request is completed (Time to First Byte + Content Download). Since a connection to the domain has already been established, there’s no need for DNS, Connect, SSL negotiation.

Request 23 - GIF file moved

Although request 23 looks quite unremarkable, there are a couple of things going on. The background of the request is yellow. This is to signify a server response status code that isn’t the usual 200. In this instance it is a 302 status code, which signifies the GIF file has been temporarily moved. In fact all responses with a 3xx status code will have a yellow background. The request details show the following information:

  • Error/Status Code: 302
Request 23 shows how WPT displays 302 redirects in the form of a highlighted bar in yellow and a different status code after the request has completed.

Notice how request 23 doesn’t require a TCP connection to be established. This is because it has already happened for this domain on request 20.

Error status codes 4xx and 5xx are displayed in a similar way, only the background is red, like in the example below (note this image is from a different test):

Request 7 shows what happens on a WebPageTest waterfall graph when it encounters a status code starting with a 4xx (error) or 5xx (Internal Server Error). The background of the request is coloured red, and the status code at the end displays 404.

The request details show the following information:

  • Error/Status Code: 404

Notice the colour of the returned resource response in this instance. Rather than being the expected purple colour for an image, it is blue signifying that it is an HTML page. Or, in other words, it is the server responding with the 404 page because the asset can’t be found.

Download chunks

Another visual peculiarity you may be curious about is the vertical stripes in each of the requests. As mentioned earlier, the lighter colour signifies the request has been made and the browser is waiting for a response. The darker colour indicates that bytes for the specific resource are being delivered to the browser. Now sometimes this doesn’t happen all at the same time. This results in what looks like zebra striping, where the browser is being drip-fed bytes over time. These are called download chunks (red arrows).

Download chunks can be seen in requests and they look like zebra striping. This indicates that the browser is being drip-fed bytes for each asset. Chunking usually occurs when lots of assets are competing for bandwidth, or the HTML early flush technique is being used.

This is very visible when an HTML early flush technique is being used (request 2 - see scenario below for more details) or if a large number of assets are being downloaded in parallel and are competing for resources (requests 3-9). Note the bandwidth graph at the bottom of the UI is maxed out from 1.6 to 2.5 seconds.

You may be asking “but this chunking also occurs after the bandwidth usage drops (2.6 seconds+), so what is happening there?”. Well the number of parallel connections has dropped so less is downloading in parallel. But the connections that have been created in requests 12 - 15 are still in the TCP slow start phase, so the the assets are still competed for (now limited) bandwidth.

Inline script execution

In the above section about A third-party JavaScript file I already mentioned how the JavaScript execution is visible if you look along a request row and identify the pink lines. But in that example a third-party script was used, which was loaded via a script tag pointing to another domain. But say for example you had a number of inline scripts in your page (Google Analytics, Dreamweaver MM_preloadImages etc). How would they be visualised?

Inline JavaScript execution can be seen as thin pink lines within the HTML download stage of the request. Said execution can also be seen on the browser main thread.

Here we see the browser downloading chunks of HTML, then in very close proximity to these downloads we see JavaScript execution lines (in pink). This is from JavaScript that is inline in the page that is executing. If you look very closely in the browser main thread you will see both HTML parsing (blue), and very small slithers of yellow to indicate the JavaScript execution.

This waterfall is also a prime example of HTML early flush which I go into more detail later in this blog post.

Different ‘First Byte’ values

In the details tab of a set of test results there is a summary table right at the top of the page. It displays some of the most important metrics for quick review. These metrics include ‘Web Vitals’, ‘Document Complete’, and ‘Fully Loaded’. To the far left of the table you will also see a ‘First Byte’ column. An eagle eyed reader may also spot a ‘Time to First Byte’ listing when you click on the pages HTML request (see example waterfall). What’s unusual here is that these two values are different, So let’s examine what is happening here.

In our listing panel for the HTML we see:

  • Discovered: 0.007 s
  • DNS Lookup: 304 ms
  • Initial Connection: 303 ms
  • SSL Negotiation: 334 ms
  • Time to First Byte: 445 ms

The ‘First Byte’ value seen in the top table is the sum of all these values (Discovered + DNS + Connect + SSL + TTFB). So it is basically the time taken from navigationStart to the browser receiving the very first byte of data from the server. It’s also worth noting that if there are any redirects during this time (not seen in the example waterfall), they will also be included in the ‘First Byte’ value. Where as ‘Time to First Byte’ in the listing panel is simply the time from request start, to when the first byte of data was received. It’s easier to explain via an annotated waterfall diagram:

Waterfall graph explaining the 'First Byte' column value, and how it is made up of separate metrics from the HTML request listing.

Common scenarios

Here’s a list of common patterns seen in a WPT waterfall chart. I’ll add more of these over time as I encounter them.


DNS Prefetch is part of the Resource Hints Working Draft. It gives a developer the ability to tell the browser that a DNS lookup is going to be needed for another domain in the near future. So instead of waiting, start the lookup immediately. By the time the domain is actually required, the browser will only need to complete the TCP handshake, and optional SSL negotiation. It looks similar to the preconnect example below, in that it is “floating” in the timeline. But here only the DNS lookup (green) is visible.

The waterfall shows the dns-prefetch occur quite some distance from where it would usually happen (with the TCP and SSL negotiation). It is also worth noting where it occurred in the timeline, directly after the HTML download completed.

Notice where the dns-prefetch occurred in the timeline: almost immediately after the HTML has finished downloading and is parsed. It is easy to see the difference if you compare it to the connection negotiations happening in requests 5 and 7, where preconnect is being used.


Preconnect is part of the Resource Hints Working Draft. It allows a developer to give the browser a “hint” that a domain will need to be connected too in the near future. By connecting to the domain early, the connection won’t need to be established later in the waterfall, thus allowing assets from said domain to be requested and downloaded quicker when they are required.

The image shows the preconnected domain establishing the connection earlier than required, before the asset is requested. This preconnect allows the request to happen immediately.

As you can see in the image above, the preconnect looks to be “floating” in the timeline. It happens way before the actual request for the image is made. This is the browser using the preconnect hint to connect ahead of time before it is required. For more information on the preconnect hint I recommend reading this blog post by Andy Davies.


Prefetch is part of the Resource Hints Working Draft. It allows a developer to tell the browser to prefetch a resource (e.g. CSS, JS, HTML document) in the current navigation, as it may be required at a later date. For example, if you know the majority of your users navigate to a specific page from your homepage (e.g. maybe your login page), you could decide to prefetch it so it will already exist in the users browser cache when it is required. In the example below I am prefetching another HTML document that sits along a users journey:

With Prefetching

The waterfall shows a request for two HTML files. One for the initial connection (request 1), another for the prefetch we have specified (request 19). The prefetched file is simply cached, it isn't parsed.

The prefetch is visible at request 19 in blue (HTML). It is worth noting that this prefetched HTML is simply stored in the browser cache. It isn’t parsed by the browser. You can verify this in the waterfall chart UI by looking at the browser main thread graph. At the point the the HTML is prefetched, there’s no main thread activity registered.

Without Prefetching

Only one HTML request can be seen on the waterfall chart because prefetch isn't being used.

WebPageTest gives us some information in the popup to let us know it is a prefetch hint:

  • Priority: IDLE (under the details tab)
  • purpose: prefetch (under the request tab)

It’s important to remember the priority of a prefetch. In WebPageTest, when testing in Chrome it is listed as priority IDLE. This maps to Lowest priority in DevTools (according to the Resource Fetch Prioritization and Scheduling in Chromium document). So a prefetch is an optional and often low-priority fetch and will be loaded as late as possible by the browser. This differs from preload which is a mandatory fetch and gets a High priority in the browser. A resource loaded using preload is layout blocking, so use it sparingly else it could actually slow down perceived performance.


Prerender is part of the Resource Hints Working Draft. It gives a developer the ability to tell a browser what a users likely next navigation could be (assuming the developer is tracking this in some form of analytics). In December 2017, with the release of Chrome 63, Google overhauled how prerender worked. Here’s a brief before and after explanation:

Pre-Chrome 63: Chrome would look for the <link rel="prerender"> element and create a hidden page for the URL listed by it. Unseen by the user, this page would be downloaded including all dependant subresources and any JavaScript would be executed. Should the user happen to navigate to the page, the prerendered page would be swapped out with the current tab, giving the user the impression of an instant page load. There were a few negatives to this method. The first, memory usage was high due to having to maintain the state of the hidden page. This made it unsuitable to low-end devices. And second, since user-facing and stateful actions should not occur with a prerender (since the user can’t see them), it was a very complex task to achieve this when the whole point of a prerender was to render the page. So the decision was taken to depreciate and remove the then current implementation.

Chrome 63+: Since the release of Chrome 63, the prerender hint is still recognised and followed by Chrome, only it is handled in a much different way. Chrome now uses a technique called “NoState Prefetch” when it sees a prerender link element. If seen it will be followed only if these two conditions are met: the user is not on a low-end device, and they are not on a mobile connection. If followed, the page will be downloaded and scanned for subresources to download. These will be downloaded and cached at the lowest priority possible (IDLE). No JavaScript will be executed on the prerendered page, and should the user actually navigate to the page, the browser will need to load the cached resources into a new browser window (not swapped out as before). A lot more information about “NoState Prefetch” can be found here.

So what does this prerender look like in a WPT waterfall chart:

With Prerendering

You can see the request for the prerendered HTML page, and the subsequent subresources loading. Many of these subresources already exist in the browser cache since they were download as part of the original homepage request.

In the waterfall chart you can see the usual page and resources loading from request 1 through to 19. Request 16 is where the prerender occurs. Here you can see a request for a second HTML page. Once completed, this then triggers requests 20 through to 29. Notice how many of these requests have a yellow background with a 304 status code. This is telling us they are identical to a resource that already exist in the browsers cache. They exist here because the homepage HTML above them (request 1) put them there only a few 100ms before. Notice how there’s very little happening in the browser main thread graph (other than the homepage parsing which is happening because request 30 (CSS) completed). This confirms that the prerendered assets and subresources are simply being stored in the browser cache for later use.

As with prefetch, WPT gives us a little information in the popup for each prerender resource to let us know the requests aren’t from a standard user navigation:

  • Priority: IDLE (under the details tab)
  • purpose: prefetch (under the request tab)

Note: It doesn’t explicitly tell us it comes from a prerender hint, only from a prefetch. Since “NoState Prefetch” is now being used, this actually makes sense.

Without Prerendering

Without the prerender link element you get your standard page load. No requests are made to other pages or subresources.

Without the prerender link element you get your standard page waterfall chart. Only requests to the current page subresources can be seen.


Preloading is a W3C Candidate Recommendation and is used to increase the loading priority of selected assets. A developer can tell the browser: “this resource will absolutely be needed soon, so load it right away”. This technique is often used when loading web fonts.

Without preload, when loading a web font the browser first needs to download the HTML and CSS, then parse both to create the render tree. Only at this point can the browser request the font. This can lead to what is known as a Flash of Invisible Text (FOIT) and Flash of Unstyled Text (FOUT). A way around this issue is to request the web font file immediately using the preload directive.

With Preloading

A preloaded request comes directly after the HTML download starts (request number 2), when the browser parses the <head>. The request for this asset is started as soon as it is discovered.

Without Preloading

The usual font loading occurs after the CSS has downloaded and is parsed. You can see the font has request number 11, verses number 2 when preloaded.

If you compare both of the images above you will see the request for the preloaded WOFF2 font is made as soon as the HTML starts to be downloaded at request number 2 (dark blue strip). The browser parsed the <head> tag, saw the preload directive and made the file request immediately.

Compare this to the second image, where the browser downloads the font after waiting for the HTML and CSS to be downloaded and parsed. Only at this point can the WOFF2 font request be made. As you can see from the image, when preloading isn’t used the font is at request number 11. I’ve written more about font preloading here if you are interested.

HTTP/1.1 vs HTTP/2

HTTP/2 is the next iteration of the HTTP protocol after HTTP/1.1. Due to the fact that HTTP/2 uses a single TCP connection and multiplexes files over this single connection, it is easy to spot the difference in the resulting waterfall charts:


HTTP/1.1 waterfall showing the stepped image requests from the page due to multiple TCP connections having no knowledge of each other.


HTTP/2 waterfall showing the simultaneous image requests by the browser. The browser will stream the image data back via a single connection.

A browser using HTTP/1.1 requests images via separate TCP connections, and this tends to happen at slightly different times (hence the stepped nature of the waterfall). A browser using HTTP/2 on the other hand requests all the images at the same time. It is the server that decides when the images will be sent back to the browser, and in what order.


Online Certificate Status Protocol (OCSP) is an internet protocol used for obtaining the revocation status of SSL certificates. One way for a browser to certify a certificate is to connect to an OCSP server for verification. When this happens WebPageTest will show you in the waterfall as seen below:


OCSP requests seen in Web Page Test appear before the request for the HTML. These request add a large amount of time to the SSL negotiation and the HTML download.

This OCSP check is bad for performance. The verification requires a DNS lookup and an initial connection to the OCSP server. Only once the certificate has been verified, can the SSL negotiation take place to the original domain. As you can see in the image, the whole waterfall is being pushed back. It takes almost 2 seconds before the HTML page can even be requested!

Without OCSP

When the OCSP doesn't happen request number 1 is the request direct to the web server, setting up the connection (DNS, Connect, TLS), then requesting the HTML file.

If you compare the with & without OCSP waterfalls, you can see that the length of the SSL negotiation is much shorter for without OCSP (300ms instead of 1000ms+) and therefore the request for the HTML file happens much quicker (at 1 second verses 1.95 seconds). The OCSP check adds 950ms to the initial HTML request on a 3G Fast connection. That’s a huge number!

If you notice this on your WebPageTest timelines you should look into enabling OCSP stapling on your server. Note: If you are using Extended Validation certificates (EV), OCSP stapling won’t fully solve the issue, see this technical Twitter thread for more details on this. Update: I’ve now published a whole blog post about certificate revocation that you can view here.

Firefox enhanced tracking protection

Firefox enabled enhanced tracking protection by default as of version 69 (June 2019). The agents on WebPageTest updated around the same time. In some rare cases the tracking protection requests could be seen in the WPT waterfalls (request 1-3):

Firefox enabled enhanced tracking protection in version 69. A DNS lookup and the requests could be seen before the initial connection in some rare cases.

According to Pat Meenan these requests should now be filtered out by default, so they will never be seen in the waterfall charts.

Service worker precaching

The use of service workers is gradually increasing, and one of the many features they allow is a fine-grain control on what assets are cached and for how long. They also give a developer the ability to precache files for future use (e.g. for offline capabilities). An important detail to remember when precaching assets using a service worker, is the browser may need to download the same files twice. Once for the HTTP cache (the standard browser cache), and again for the service worker cache (Cache API). Theses are two totally separate caches, and don’t share assets. These duplicate requests can be seen in a WebPageTest waterfall chart:

The two sets of identical assets are downloaded twice, once for the HTTP cache then again for the service worker cache.

In requests 17 and 18 you can see the service worker JavaScript being requested, downloaded, and initialised. Immediately after this, the service worker looks through it’s precache JSON file and requests any assets listed. Note: In the example above the Workbox library is being used to simplify the service worker workflow.

Identifying requests not from the main page document

I mentioned directly above that you could (potentially) identify service worker requests by examining the shape of the waterfall. In the example above there is an obvious ‘step’ where the main document requests finish, and the service worker precache requests start. But waterfalls from some sites are complex beasts and this shape isn’t always visible. So a new feature has just been added, where requests that don’t originate from the main page document (e.g. <iframe> and service worker requests) are now highlighted in blue (Chrome only). So it is now incredibly easy to spot when these requests are being made! I believe we all have Andy Davies) (and of course Pat Meenan) to thank for this new functionality.

Resuests that don't come from the main thread are now highlighted in blue.

In the above example we see the simple waterfall from the excellent Squoosh image compression progressive web application. Request 10-12 are now highlighted in blue as they all have a different ‘Document’ value (Document: in the ‘Details’ tab when you click on each request.

Chrome stair-step

Chrome has included a prioritisation technique that is named due to the pattern it creates in waterfall charts. It involves Chrome examining the assets in the <head> tag (before the page even has a <body>), and requesting, downloading, and parsing these requests first. The browser even goes so far as to delay requests for assets in the body until the <head> requests are complete. It is easier to see this stepping in a HTTP/1.1 graph, as seen in the example below (although it also occurs in HTTP/2):

Chrome stair-stepping occurs when the browser delays requests from the body element in favour of completing the requests from the head element. This can be seen in the WPT waterfall as a distinct step in the timings.

In the above image from the BBC News website, 8 out of the first 9 requests are made for assets in the <head>, with only 1 request to a JavaScript file located in the <body>. The “step” isn’t very long in terms of duration, only around 200ms. But it gives the browser enough time to concentrate all CPU and bandwidth on downloading and parsing these assets, so the <head> is then setup and ready to go before the <body> assets are downloaded and parsed.

Not much has been written about this “layout-blocking” phase in Chrome, but it can be seen in detail in the Resource Fetch Prioritization and Scheduling in Chromium document by Pat Meenan, and also in Chrome’s resource scheduler source code.

HTML early flush

HTML early flush was mentioned above in the download chunks section. It is when a web server sends a small slice of the HTML document, before the whole HTML response is ready. What this then allows the browser to do is parse the HTML it has received, and look for assets that it can then request early (compared to waiting for the whole HTML document to download, parse, and then request).

With early flush

With early flush the server sends a small chunk of HTML to be parsed. This parse then triggers additional requests much earlier than when it would usually happen (and therefor speed up the website).

In the example above, the small download chunk of HTML the browser receives (request 2) contains the <head> tag which links to JavaScript, fonts, JSON, preload directives, and dns-prefetch resource hints. This HTML is parsed and 16 requests are almost immediately triggered very close to each other. Note: Notice how I didn’t list CSS. has inlined the CSS in a <style> tag (so no request to a CSS file is required). They then trigger CSS requests via JavaScript later in the waterfall once the JavaScript has loaded and parsed.

Without early flush

Without early flush the browser must wait for the whole HTML document to be downloaded before it can be parsed, and then additional requests can be made. Notice how the HTML download complete lines up vertically with the request being made directly after it.

If you compare the waterfalls for with & without early flush (from different sites unfortunately): With flush you will notice how the requests for assets is made during the HTML download response. Compared to without, where the browser must wait for the whole HTML document response to complete. Only then can it be parsed and requests made to other page assets.

Flushing allows the browser to make requests earlier in the waterfall, and therefore makes the page load and feel faster (if implemented correctly).

Long DOM content loaded (DCL) time

The vertical lines on the waterfall chart are usually very thin, maybe only a couple of pixels thick. This shows they take very little time to actually happen. But in some cases the DOM content loaded line takes up a large chunk of time, and you will very easily see it on your waterfall chart! Take a look at the two (different) charts below:

Normal DCL

DOM Content Loaded (DCL) vertical line is very thin, maybe only 1-2 pixels wide showing it took very little time.

The pink DOM Content Loaded (DCL) vertical line is usually almost instantaneous. Very thin and barely visible on the waterfall chart as it usually occurs around the same time as other events like DOM Interactive & Start Render. This is what a healthy DCL looks like.

Long DCL

DOM Content Loaded (DCL) vertical line is very thin, maybe only 1-2 pixels wide showing it took very little time.

Now compare the previous waterfall chart to the one above and you can easily spot the issue. The DCL line is covering almost 1 second on the waterfall chart! That’s not a healthy looking line. In the chart you will see some unusual goings on that can help explain what is happening. The light pink line on request 4 is showing a lot of JavaScript execution is happening, and it just so happens to be the exact same width as the expanded DCL line. The browser main thread is solid yellow, which confirms that a script is parsing. The CPU graph is at around 80-90% capacity over the same time period and the page is blocked. Curiously the “Bandwidth In” graph is hardly registering anything, so whatever is causing the issue has already downloaded and is being parsed.

So what is actually happening? Well, there’s some very expensive JavaScript running in the file at request 4. Often this is caused by JavaScript attached to the jQuery $(document).ready() method (or some equivalent). So if you ever see a DCL line eating a large chunk of your waterfall chart, you now know where to look.

The never-ending waterfall

Occasionally if you have a third party script setup incorrectly, you can end up in a situation where a test will take a very long time to stop, as can be seen in the waterfall chart below:

A third party script can keep the network open and increase the amount of requests by thousands.

Yes you are seeing that correctly, 6150 requests logged by WebPageTest! So many in fact that my browser struggles to render all the DOM elements on the page. This is where the ability to customise the waterfall really helps, as you can limit the number of visible requests to a reasonable number.

So what exactly is happening? Well there’s a clue in the network activity graph. If you look closely you will observe that it never reaches zero. WebPageTest waits for there to be no network activity 2 seconds, at which point it ends the test. Unfortunately this third party script is executing every 30ms (request 4) and downloading a mere 43 bytes of data each time. Just enough to keep the network alive. It’s also worth noting the CPU utilisation graph, which is maxed out over the time period (120 seconds). That’s what happens to a device when a script is executing 33 times per second.

At 120 seconds WebPageTest automatically forces the test to stop. There are a couple of ways to force a test to stop yourself (other than fixing the issue):

So if you see unusually long tests running, check to see what your third party scripts are doing to the network graph.

Rogue image downloads (HTTP/2 with SRI)

I personally ran into this issue very recently on GOV.UK. The waterfall was showing an image download that at first glance shouldn’t actually be able to be downloaded:

On request number 2, the image is downloading from the assets domain before the connection has been established.

Request number 2 shows an image downloading before the connection to the domain has been negotiated. I’ve now written a whole blog post “HTTP/2 and Subresource Integrity don’t always get on” about this curious case if you want to delve into the details. But a quick TL;DR for you here is: the connection to the “assets” domain in request 2 is using HTTP/2 connection coalescing, so it is using the same TCP connection from request number 1. The requests 3-10 are waiting for an crossorigin="anonymous" connection because Subresource Integrity is enabled on the CSS and JavaScript, hence why it looks strange to be receiving a file from the domain, and making the connection at the same time.

Requests with no response body (Error/Status Code: -1 & -2)

There’s more status codes that those mentioned in the Request 23 - GIF file moved section, another two available are codes -1 & -2:

Waterfall requests that return with no response body come back with an error code of minus one.

Notice how the above image is requesting assets from the third party domain then immediately failing. It is also possible for this to happen on requests from the same domain, as seen below:

Waterfall requests that return with no response body come back with an error code of minus two as well.

When a request is made but WPT sees no response, it is given the error code of minus 1 or minus 2. This can either mean the response was never sent back by the server, or WPT simply didn’t detect it. This can also be seen if you click on the individual requests:

  • Error/Status Code: -2
  • Content Download: 0 ms
  • Bytes In (downloaded): 0 B
  • Uncompressed Size: 0 B
  • Bytes Out (uploaded): 0.5 KB

There a little more information from Pat in these forum posts Error code -1 and Error code -2 if you are interested.

Third party blocking JavaScript

Back in the early 2010’s there was a recommendation that you should use a third party CDN provider to serve common JavaScript libraries (jQuery, Backbone, Underscore etc). The theory being that as a user navigates the web, these common libraries would be cached by the browser and reused for different websites since they are coming from the same third party domain. Unfortunately things are never that simple. With multiple CDN providers, a huge number of library versions available, aggressive browser caching, and double-keyed HTTP cache, the theory never really panned out. You can see the effect of this externally hosted JavaScript quite clearly in some waterfalls:

The third party JS from the CDN is very visible on the waterfall chart, assets downloads are delayed until it is downloaded and executed. Pixels are only painted to the page after this point too.

There’s a fair amount going on in this waterfall chart. Request 13 is the important one to focus on. This is the browser requesting a version of jQuery from the Google Hosted Libraries CDN. The connection takes a while to negotiate, then the download takes a chunk of time because bandwidth is being used for other requests (11, 12, 14). Only once the script has downloaded does a whole bunch of other activity start. The JS is executed and there’s a 2nd HTML parse by the browser, which suddenly triggers lots of requests for images. Notice where the “Start render”, “RUM first paint”, and “Page is Interactive” green graph appear on the chart. They are all being pushed back along the waterfall because of this single third party script request.

And that’s why it is now recommended you Self-Host Your Static Assets. This example was sent over by Mike Herchel and if you are interested in what happens once you remove the third party JS, check out his thread here.

HTTP/2 prioritisation

HTTP/2 is different to HTTP/1.1 in the fact that it aims to use a minimal number of TCP connections (1 or 2). Whereas for ‘optimal’ HTTP/1.1 performance the browser opens 6 TCP connections to a single domain, thus allowing 6 assets to be downloaded at a time, HTTP/2 is different. It introduces multiplex streams over a single TCP connection. A very simple explanation of this is:

  1. The browser opens a TCP connection to the server and downloads the page HTML
  2. Browser parses the HTML and looks for all other page assets (CSS, JS, Fonts, Images etc)
  3. Browser sends a list of assets it needs to load the page to the server, and the priority it would like them in
  4. It is then up to the server how it delivers these assets to the browser and in what order (ideally using the browser prioritisation as a reference). This is where HTTP/2 prioritisation comes in
  5. Server then sends multiple assets at the same time via streams across a single TCP connection

How the assets are prioritised depends on how the server has implemented the HTTP/2 specification. A lot more details about H2 server / CDN prioritisation check out Andy Davies’ ‘http2-prioritization-issues’ repository which is used to track results from Pat Meenan’s ‘HTTP/2 prioritization test’.

The test works by first warming up the TCP connection (seen in request 2 & 3). TCP slow start is part of the congestion control strategy used by the TCP protocol. It is used to stop the server flooding the TCP connection. The server gradually ramps up the speed, while looking to see if any packets are lost along the way. If packets are lost, it drops the speed.

Once the connection is up to speed the browser then requests 30 images concurrantly at a low priority (request 4 to 33). When two low priority images have downloaded (and therefore the network buffers are filled), then it requests a high priority image. The priority of the image is sent to the server along with the request (if the browser supports it), and is decided by the browser using the rules set out in the ‘Resource Fetch Prioritization and Scheduling in Chromium’ document. Looking at this document we can see that in Chrome an ‘Image (in viewport)’ in the ‘Load in layout-blocking phase’ is given a ‘High’ priority. Once one high priority image has downloaded, then the browser requests another.

The test has been designed to see how the server copes with H2 prioritisation when it is given a higher priority file to send, while already sending low priority assets. A server that prioritises correctly should divert resources from low priority assets, to high priority assets (so the high priority assets are loaded first).

So let’s take a look at a waterfall from good and bad prioritisations, and discuss what is happening:

Good - Fastly

In the waterfall we can see the Fastly CDN reprioritizing the stream when the browser notifies it of high priority images (request 34 and 35).

So what exactly is happening in this waterfall? Requests 2 and 3 are warming up the TCP connection. Requests 4 - 33 are images requested with a priority of ‘lowest’. Keep in mind that the lower the request number, the faster it was discovered and requested by the browser, e.g. image at request number 4 was seen, and requested before the image at request 20. If the server didn’t bother with any form of prioritisation and simply sent images on a first come, first served basis, images later in the waterfall would always be delivered last, since they would always be at the back of the queue. But what we are actually seeing is the server sees a request for the high priority images and it stops streaming the ‘lowest’ priority data in favour of ‘high’ priority data, so the high priority images are completed sooner.

Now if you compare this to a CDN with poor HTTP/2 prioritisation:

Bad - Level 3

In the waterfall we see all the lowest priority images download first, before high priority images are downloaded (request 34 and 35).

Here we can see the server essentially ignoring the information it’s been given in the request about the high priority images. The two high priority images are added to the end of the download queue. The server continues to send low priority data and makes no allowances for the fact that by doing so, it is delivering a suboptimal experience to the user. You can actually see this in the waterfall by looking at the green ‘start render’ line: for good prioritisation this is at approximatly 9 seconds, for bad prioritisation it is almost 21 seconds!

For more information on H2 prioritisation and many other H2 topics check out Barry Pollard’s book ‘HTTP/2 in Action’.

Large Time to First Byte (TTFB)

Time to First Byte is the time it takes from the browser requesting the first asset, to the time at which the server sends back the first byte of data to the browser. It consists of:

  1. Browser requesting the asset from the server (after DNS + Connect + SSL)
  2. Time taken for the packets to travel from the browser to the server
  3. Server recieves the request. It then processes and constructs the response and transmits it back
  4. Time taken for the response packets to travel from the server to the browser

The time taken to travel from the browser to the server is known as network latency. A data travelling there and back again is known as a Round Trip (RTT). So how do you use WebPageTest to identify when you have a large TTFB?

In the graph the TTFB is large, at 3.1 seconds. With very little happening on the CPU graph, and nothing happening on the bandwidth graph.

The above test was run on a standard ‘Cable’ connection as defined by WebPageTest, so the connection has a RTT of 28ms. Now if you compare this value to the TTFB, which is 3.1 seconds, we can see there’s an issue here. By looking at the connect negotiation time (in orange on request 3), this gives you an indication of how quickly packets can travel across the network (32 ms in this case). So it’s clear to see that the TTFB delay isn’t caused by congestion over the network. There is zero activity over the network according to the bandwidth graph, and fairly minimal activity happening on the CPU. To the device is waiting on the server to respond before it can do anything else.

In this case it’s the processing time on the server that is causing the delay. Whatever the server is doing to construct the response is taking approximatly 3 seconds. That’s a huge amount of time to construct the HTML. There are far too many reasons to list as to why this could be happening on the server, but a good starting place would be to look at the databases queries, hosting settings, server resources available, and the server software that is running. Whatever is causing the issue needs to be identified and fixed, as this will be having a huge impact on the sites users. So if you see a WebPageTest waterfall that looks like this, examine your server setup and try to reduce this time. As Harry Roberts mentions in his article ‘Time to First Byte: What It Is and Why It Matters’:

While a good TTFB doesn’t necessarily mean you will have a fast website, a bad TTFB almost certainly guarantees a slow one.

Inactivity after SSL negotiation

This is quite an unusual waterfall. We see a large Time to First Byte on request 3, but we see a huge gap with apparently no activity at all:

An unusual waterfall in the fact that there is a huge gap between the initial SSL negotiation and the HTML request.

What is interesting about this waterfall is we can see the DNS + connect + SSL negotiation happen very quickly (34ms + 33ms + 80ms respectivly), then there’s zero activity on the waterfall, CPU and bandwidth graphs over this time period. This indicates that the device is idle and waiting on work to do. Right towards the end of this period of inactivity we see the browser instigate two OCSP revocation checks. But according to the waterfall the SSL negotiation has completed by this point in time.

I can’t be 100% sure why this is happening (it’s an old test with no tcpdump log to examine the network activity), but if I were to guess I’d say that there is something unusual happening with the sites certificate. It could be there’s an additional OCSP check happening that isn’t being displayed on the waterfall, or maybe the SSL negotiation hasn’t completed properly and the browser is trying to recover from the error. But given the fact that nothing is happening on either the CPU graph or the bandwidth graph, whatever is happening isn’t very work intensive on the device. It is worth noting that the site currently uses an Extended Validation (EV) certificate that Chrome will always conduct an OCSP check on. If anyone has any other ideas about what is happening in this waterfall, I’d love to hear them, so let me know.

Missing component times (DNS, Connect, TLS)

This should no longer be an issue on WebPageTest, but it is worth pointing out if you ever look at waterfall test results from iOS devices pre-iOS 10. iOS versions before 10 didn’t report any of the connection component times back to WebPageTest. This was a limitation of what iOS reported through the Safari developer tools interface. Heres a test from 2016 on an iPhone 5c running iOS 9.3:

Older versions of iOS didn't report DNS, Connect and SSL times back to WebPageTest.

As you can see the information about DNS, Connect, and SSL are all missing from the waterfall. Note that although not reported individually, the times are included in the TTFB metric so it is still correct. Now compare this to a test run in 2020 on an iPhone 5c running iOS 10.3 (the last version supported on this device):

The iPhone 5c on 10.3 now reports the individual component times.

All iOS devices currently running on WebPageTest report the DNS, Connect, and TLS times correctly. All of them are running iOS >= 10.3. NOTE: There currently (Jan 2020) seems to be a rendering issue on WPT with the iPhone 6 and iPad Mini 2 that was introduced in iOS 12.2, whereby the test is rendered midway through the waterfall chart. But by looking at the two tests listed it is possible to see some component connection information rendered. So at least that isn’t broken…

The impact of a CPU bottleneck

This is an old test but it gives you an example as to what can happen once the CPU becomes a bottleneck. You can see the full WebPageTest run here. It is filled with lots of sporadic short bursts of activity across the whole page load. I’ve focussed on the first 20 requests in the diagram below as the full chart is far too long:

In the waterfall we can see connections fully negotiated then activity simply stops. Looking at the bandwidth graph we see very little network activity, but the CPU activity is at 100%. This is killing all network activity.

As you can see from the graph, there are multiple TCP connections negotiated in requests 2-5, 8-10. At the same time take a look at the bandwidth graph, it starts to increase then suddenly drops out. If you look at the CPU graph you can see it jump from 10% to 100% in ~200ms at around the same time, then it stays at pretty much 100% for the rest of the page load. This CPU usage is stopping all other process’ from doing their work. You can read the full response as to what exactly is happening in this forum post, but a TL;DR is: a combination of multiple SSL negotiations at the same time (they are CPU resource intensive) and heavy CPU usage caused by the parsing of applications JavaScript. Both combined with a testing machine that is underpowered leads to waterfalls that look like the one seen here.

Consequences of a 301 redirect when changing a URL

You sometimes come aross sites that like to either remove the www from the start of their URL, or others like to add it back in. It’s easy to spot this on a WebPageTest waterfall chart as it usually happens in the top few requests, highlighted in yellow with a status code after the total time of the request (see the section above ‘Request 23 - GIF file moved’ for more information). Below you can see the impact this can have on a waterfall chart for If it is present, Twitter will remove the www from the URL when you hit the site. Both tests were using Chrome on a 4G connection:

With WWW

The user has entered '', in response the browser gets a 301 redirect to remove the 'www', then continue as normal.

In the above waterfall the user has requested ‘’. Request number 1 is the browser requesting ‘’, the Twitter server responds back and tells the browser to remove the ‘www’ and then try again. With the connection already established, the HTML is requested as soon as it recieves the 301 redirect. Notice the times highlighted in the image:

  • HTML requested at 800 ms
  • RUM First Paint at 1165 ms
  • Start Render at 1200 ms

So what does a waterfall look like when a user enters ‘’ directly into the browser address bar (or it is auto-completed)?

Without WWW

The user enters the URL as '', so no 301 redirect can be seen and the HTML downloads much quicker.

When the user enters the URL that the server agrees not to modify, no 301 redirect can be seen in the waterfall chart. Request number 1 follows the usual connection process (DNS + Connect + SSL). Because there’s no wait time for the server to respond with a 301, this has quite an impact on the web performance metrics:

  • HTML requested at 536 ms (264 ms quicker)
  • RUM First Paint at 900 ms (265 ms quicker)
  • Start Render at 1000 ms (200 ms quicker)

It just goes to show, there is a performance impact associated with 301 redirects, so keep that in mind when using them.

The above waterfall charts are testing using a modern browser (Chrome). Let’s see what the same waterfall chart looks like in a legacy browser. Things don’t look quite as performant:

IE10 does a full connection negotiation for both the 'www' and the 'non-www' domains. Mix that together with a redirect and a OCSP revocation check and you have a pretty poor Time to First Byte (TTFB).

Here we see IE10 on a 4G connection. There’s a whole bunch going on with request number 3. First we have a full connection negotiation (DNS + Connect + SSL), then we have a pause for the OCSP revocation check (requests 1 & 2), the browser then waits for a response back from the server to remove the ‘www’. Then quite unexpectedly (to me) it does another full connection negotiation, this time to ‘’! The browser seems to see ‘’ and ‘’ as totally different websites. Not very optimal for web performance. Thankfully IE10 usage is now pretty much non-existant and the issue looks to have been fixed in IE11!

The missing intermediate certificate

How can you tell when one of your intermediate certificates from your Chain of Trust is missing? Well, assuming your SSL certificate contains Authority Information Access (AIA) information, you will be able to tell from a waterfall chart:

If the intermediate certificate is missing from the chain of trust, the browser will attempt to go and fetch it before the SSL negotiation can complete. This can be seen on request number 1, with a file request for a *.crt file.

Usually you would host the intermediate certificate along with your leaf certificate. But if for some reason you forget, the browser will attempt to retrieve the missing certificate from the information provided in the sites leaf certificate. As you might expect this isn’t great for security or web performance. If the certificate can’t be retrieved, then the browser may need to ‘soft-fail’ and just assume that the leaf certificate is legitimate. Also, while the intermediate certificate is being retrieved (a process that is synchronous and blocking) the SSL negotiation can’t complete. During this period the user sees a blank page. So remember, always check that your sites certificate chain is complete using a tool like SSLLabs. If you want to know more about certificate revocation in browsers, I’ve written a blog post here that you may find interesting.

A vary unusual waterfall (empty gaps between download data)

This was such an interesting and frustraiting issue, I’ve actually written a blog post all about it which you can find here. But a TL;DR; version is: notice how the requests 6 and 7 have a weird set of gaps between the data chunks:

Annotated waterfall, with exactly what is happening (see following paragraph)

Esentially what is happening is the browser is recieving data from a CDN that has been poisoned (it is missing the CORS headers needed for the font fetch). Lets step through the waterfall:

  1. First the WOFF2 font files are requested by the browser after a new TCP connection is established
  2. A very thin slither of data is received (most likely the headers)
  3. The browser immediately cancels the request as there are no required CORS headers. This explains the ‘empty space’ between the data.
  4. The browser immediately requests the next fonts in the @font-face declaration (requests 8 & 9)
  5. The browser still receives chunks of WOFF2 data because this data was already in transit over the network or sitting in one of many buffers along the way.

And what’s the solution to this issue? Make sure you are serving vary: Origin along with your font responses. Simple fix when you know that huh!

Examining undefined 204 status codes

In this chart we see multiple requests going out to the server, but as you can see the responses back are quite unusual. The colours for requests 14, 16-21 are all grey indicating they are of the type ‘other’. Clicking on each of the requests shows Error/Status Code: 204 and the bytes coming in were 0. Looking up the status code 204 details, that makes sense. Status code 204 indicates that there’s no content. The request has been successfully completed by the server and there’s no aditional content to send in the response payload body. In fact if you read the specification a 204 response cannot have a response body. The response terminates once the first empty line is recieved after the headers.

Waterfall showing undefined requests to the server that are recieving a 204 status code. These are icon requests that are failing because they aren't on the server.

So what’s happening here? In hindsight, once you know the answer it’s quite obvious, but very few clues are actually given in the waterfall responses, so it can be very frustraiting to debug. Examining the ‘start’ and ‘end’ responses that are top and bottom of the ‘undefined’ responses, shows the browser requesting icons and the server is responding accordingly. But the icons being requested between the successful responses are coming back as ‘undefined’, as it could be that the icons don’t exist on the server. Now I have struggled to reproduce this waterfall in my own tests. I can get a 404 response by removing an icons src key in the manifest.json file, and a 304 response by setting the src to "" or "/". But a 204 is something I haven’t managed. So why exactly this particular server setup responded with a 204 rather than a 404 is a mystery to me. Any thoughts or ideas please do tweet me!

So if you see this pattern in one of your waterfalls make sure your icons are all referenced correctly and exist on the server.

Service Worker precache slowing a page load

Sometimes a Service Worker can impact a pages web performance and actually make it slower. Service workers have the ability to precache assets to make future navigations more performant, or allow a website to work offline. But you need to be careful where this precaching occurs in the standard page load process. In the waterfall below we can see the service worker registering too soon and instructing the browser to load additional assets into the Cache API. This is a different cache compared to the standard HTTP Cache.

Waterfall showing what can happen when a service worker isn't loaded at the optimal time, leading to page assets competing for resources.

Lets step through these requests as there’s a lot going on in this waterfall:

  • Requests 1-13 are initialised by the main HTML page (text in black).
  • Requests 14-15 are downloading the pages service worker (text in blue). Once registered, the service worker queue’s up additional assets to be downloaded (21, 23, 25-28, 37-41).
  • Notice how request 38 is downloading at the same time as the original page assets in requests 42-49. These assets are now competing for the limited device resources.
  • Between 2.4 - 2.6 seconds the device is both CPU and bandwidth limited. The script from row 38 is causing huge amounts of script execution, as seen in the ‘Browser Main Thread’ graph at the bottom.
  • Notice how request 3 and request 38 have identical filenames and similar JavaScript execution patterns. This is the same file being downloaded once into the HTTP Cache and then again into the Cache API.
  • There are even more request rows after 54, so even at this point the page has yet to complete loading.

The fact that the pages service worker has fired too early has caused the browser to reprioritise its asset download queue in such a way that it will slow down a page load due to limited bandwidth and CPU time. The service worker assets are stealing resources from page assets before the page has finished downloading!

So what’s the fix for this issue? Well it’s actually quite simple and is mentioned in this service worker registration primer. Wrap your service worker registration code in an event listener and delay registration until after the pages load event fires. This delay allows the pages assets to download, which should allow the page to (hopefully) render quickly for a user. At this point the user can start interacting with the page and the service worker can register and precache any assets in the background.

if ('serviceWorker' in navigator) {
  window.addEventListener('load', function() {

In regards to the repeated download into two sets of caches (request 3 and request 38), if you are using Workbox, check out the ignoreURLParametersMatching setting on this page. The option lets you configure a pattern for hashed files that can safely be served from the HTTP cache (if present), which lets you avoid double-downloading them. Thanks Jeff Posnick for the tip!

Credit for bringing this waterfall to my attention goes to Andy Davies, who recently spoke about this issue. Slides for his talk can be seen here (slide 50 to 62). I hear there’s a whole blog post all about this issue in the works too, so watch out for that.

More to be added soon…

As I discover more common waterfall scenarios I will add them here. If you know of any common ones that are missing, please do let me know!

Hidden gems

So there are features that are hidden in plain sight on the WebPageTest UI that you may not have even noticed before. Here are a few that I’ve found useful:

Now it may seem obvious to some, but it is worth pointing out nonetheless. There is a direct relationship between the filmstrip view and the corresponding waterfall chart below it:

The filmstrip and the waterfall chart are related. There's a red line to the left of the waterfall that corresponds to the red line that moves across the waterfall chart as you scroll. You also see thick orange borders around each image if something has changed on the screen.

To the far left of the filmstrip you will see a 1px vertical red line. As you scroll the filmstrip horizontally you will see an identical red line moving across the waterfall chart. These are directly related. Together they show you what the page looked like at that exact point in the waterfall chart (you would not believe how long it took me to notice this feature!).

Another feature that can be seen is the use of a thick orange border around some of the images. This orange border signifies that something has changed on the screen (compared to the previous one). This is very useful if you are trying to identify even minor changes between screenshots (like an icon loading).

You can see both of these features in action in the screenshot. There is a thick orange border around the image at 0.9s, as it shows a major change to the page compared to the image at 0.8s. Looking closer at the waterfall we can see the red line is approaching and the vertical green line (start render). The image at 0.9s is actually the start render time for the page.

What do the filmstrip thumbnail border colours signify

Over the past month or so (May/June 2020) there’s been an important development for web performance on the web. From 2021, Google will start evaluating page expereince and will include the results in how a page is raked by the search engine. They will be evaluating a pages usability using three metrics (at the time of writing). Together they are known as the ‘core web vitals’:

  • Largest Contentful Paint (LCP) - Measures loading
  • First Input Delay (FID) - Measures interactivity
  • Cumulative Layout Shift (CLS) - Measures visual stabiliy

You will be excited to hear LCP and CLS are now very easy to spot on the filmstrip view, thanks to Rick Viscomi who raised this issue and of course Pat Meenan who implimented it. A key of what each of the borders means can be seen below:

  •   No border: no visual change occurred.
  •   Yellow border: a visual change occured on this frame.
  •   Red border: Largest Contentful Paint (LCP) occured on this frame.
  •        Yellow/black dashed border: There was a visual change and a layout shift on this frame.
  •        Red/black dashed border: Largest Contentful Paint (LCP) and a layout shift occured on this frame.

Or if you prefer, an annotated filmstrip with all border variations can be seen below, generated from this comparison view.

The filmstrip view thumbnails will have different borders depending on what metrics changed in that frame.

How to generate customised waterfall chart images

Almost all of the waterfall images in the article you see above have used this feature that is tucked away at the bottom of every waterfall chart. WebPageTest gives you the ability to customise what is included in a waterfall chart:

The 'customize waterfall' link leads you to a page with a whole host of options you can use to customise the waterfall image.

Clicking the ‘customize waterfall’ link directs you to a set of customisation options (see image below):

The 'customize waterfall' link gives you the ability to select what requests and graphs you want to include on the output image, the time the waterfall covers, as well as it's dimensions. These are just a few options available.

As you can see in the image above, I have customised the output image. We have:

  • Set a custom image width
  • Set a custom time that the whole waterfall chart covers (note this will lead to cropping of requests further down the waterfall)
  • Only selected certain requests to be shown either individually or as a range (notice the ellipsis between these that identify missing items)
  • Unchecked ‘Show CPU Utilization’ so the ‘CPU Utilisation’ and ‘Browser main thread’ graphs are hidden

All these options allow you to remove much of the noise in the chart so you can focus on the areas you wish to identify and highlight. Once you are happy with the image, simply right click and save the image to your device as you would any other image.

How to add custom marks using the User Timing API

Here’s a useful feature you may not know about. Using the User Timing API you have the ability to mark certain points in your page load. These points will be discovered by WebPageTest and displayed accordingly in the results.

For example, if you wanted to know when the browser got to the end of the <head> tag, you could add the following code right before the closing tag:

<!-- head stuff here... -->

The browser then sets a mark at this point which can be read by WebPageTest. The resulting WebPageTest run will show you results similar to this:

Adding custom marks via the User Timing API gives output metrics similar to this in the WebPageTest output.

As you can see from the image above I have set four User Timing marks on the page and one of them is called mark_head_parsed. You can add as many marks as you need, it really depends on what you are trying to measure. Now if you head over to the ‘customise waterfall’ link (as mentioned in the section above), you will see one or more purple triangles and vertical lines on the waterfall chart. These are the User Timing marks we just set:

The custom timing marks can be seen in purple on the waterfall chart if you customise the waterfall image.

In the image above the timing marks are now visible (you get the option to toggle them on and off if required). So you may be thinking: “So why can’t I see these marks on the interactive waterfall chart?”. The reason for this is because they have been disabled by default on the interactive chart. Many third-party JavaScript scripts were including marks in their code, which was polluting the graph with a whole load of noise that were irrelevant to most WebPageTest users. So it was decided to disable the visibility of the marks by default. Marks can now only be seen within the ‘customise waterfall’ link.

Note: If you happen to be running a private instance of WebPageTest, you can configure it to display the User Timing marks to be on by default for the interactive waterfall chart.

How to display the bandwidth graph on mobile devices

When you run a test on one of the real mobile devices that are sitting in Pat Meenan’s basement in Dulles, VA, by default you don’t get a bandwidth chart like you do on desktop agents. Resulting in an output that looks like this:

Without tcpdump enabled the bandwidth graph isn't visible at the bottom of the UI on mobile devices.

If you do want to capture this data and display the graph in the resulting test, make sure you check ‘Capture network packet trace (tcpdump)’ under the ‘Advanced’ when you configure your test:

The tcpdump option is located under the 'Advanced' tab when you configure your test.

This will give you a slightly different version of the ‘Bandwidth In’ graph than you see on desktop agents:

Enabling tcpdump now displays the bandwith graph on mobile devices, which gives you information on Bandwidth In and Duplicate (wasted) Data.

It now gives you information about the data wasted due to the retransmission of packets over the network (although not actually seen on the example graph). An example with ‘Duplicate (wasted) Data’ can be seen here along with a little more information.

How to test a 404 pages web performance

So I had a weird usecase for wanting this feature in WebPageTest releated to one of the GOV.UK applications, so there may be others out there wondering about their 404 page performance (right?). So it seems like a ‘feature’ worth documenting. Keep in mind that many automated testing tools will simply abort a test should you happen to run one on a 404 page This makes sense after all as the tool is encountering a 404 status code, why would it continue?

But my thoughts on this are: should your user mistype a single charecter in a URL, they are likely going to be hitting your 404 page. What if you happen to have some strange server / templating configuration for the 404 page that is serving a large amount of data. It could even just be just a large image on the page. Do you really want your users to be wasting their time and bandwidth on a page they don’t even want to be on? A secondary point is that a 404 request will be hitting your origin server too (including the servers hard drive), so you want it performing optimally. And remember to ‘Make sure you have a favicon’, else there will be a lot of hits to your 404 page! I’ve now written a blog post all about 404 performance testing which you can read here.

There is now a workaround in WebPageTest that allows you to do this, either via the user interface, or WPT scripting:

User Interface

Force WPT to measure the performance of a 404 page by setting a minimum test duration.


setMinimumStepSeconds 2

Simply set a minimum test duration of a number of seconds longer than the 404 page takes to load to gather metrics about your 404 page performance.

How to delve into the download chunk data

I mentioned the fact that download chunks are visible in each row, and they are identified by the darker colour in the Download chunks section before. But lets say you wanted to find out a little more about these chunks. Simply click on the request row you want to find out more about then click on the ‘Raw Details’ tab. Scrolling down a little in the box you will find a ‘chunks’ key within the JSON data. The ‘chunks’ key has an array as a value, and within the array there are separate objects each with information about a single chunk of data.

There's detailed information about each requests download chunk data hidden within the 'Raw Details' tab when you click on request row.

Each chunk object has two keys:

  • ts: stands for timestamp. It’s the point at which this download chunk completed (not started). The time is in milliseconds and relative to the start of the waterfall
  • bytes: pretty self-explanitory, the number of bytes that were downloaded in this chunk.

WebPageTest works backwards from the completed chunk timestamp, looking at the bandwidth available at that point in time and calculates how long it would take to download that number of bytes. This gives a chunk its width on the waterfall. This is all measured from Packet Capture (pcap) bandwidth information.

Multiple chunks in very close proximity merge together to look like a single large chunk, but this data allows you to drill down into each large chunk to see the individual chunks.

How to view a sites HTTP/2 dependency graph

So HTTP resource prioritisation is a notoriously complex beast of a problem (that’s an understaitment!). Don’t believe me? Here’s Robin Marx giving a 35 minute talk all about it at FOSDEM 2019. Now what you may not realise is that if a website is using HTTP/2 and you run it through WebPageTest, there is a way to quickly view the HTTP/2 dependency graph for the particular browser used in the test:

The 'View HTTP/2 Dependency Graph link is situated directly under the waterfall view chart.

I’m not going to go massively into the details of what is involved in prioritisation, there are plenty of books, blog posts and talks online from folks who know much more about it than I do. But I will try to give a brief overview of how to read a basic chart. It’s worth noting that each browser does this differently:

  • Legacy Edge and Internet Explorer don’t support prioritisation (they load everything split equally in parallel)
  • Safari loads all resources in parallel, but splitting available bandwidth depending on the priority as Safari sees it
  • Firefox builds a complex dependency tree and loads resources according to it
  • Chrome (and all Chromium-based browsers) construct a linear list and loads according to that list

There’s an excellent blog post here from Pat Meenan looking at the impact this prioritisation can have on actual user percieved performance. For the annotated dependency graph below I’ve choosen the Chrome browser and the website GOV.UK as a test subject, since I know it now runs on HTTP/2.

The Dependency graph for GOV.UK from a Chrome browser.

This is quite a busy looking diagram so I will pull out a few features:

  • Each of the boxes corresponds to an individual resource, and the colour of the box corresponds with the key colours defined above the WPT waterfall chart
  • Top left of each box is a number that corresponds to the request number in the waterfall chart
  • Each resource is weighted by Chrome. The higher the number, the more important Chrome regards it
  • Multiple ‘trees’ indicate multiple TCP connections used to download a sites resources
  • Resources on the left are requested first, moving across to the right for later requests
  • Resources from top to bottom indicate the dependencies. A file lower down the list depends on those above before downloading

Where you see a ‘fork’ in the tree (after request 10), this is Chrome shifting priorities as resources are discovered. Chrome uses a linear list to prioritise the download order, and WebPageTest only provides a single (complete) view of the tree. If a higher priority request is discovered it is then inserted into the list by giving it the parent of the next highest priority request. HTTP/2 will then move everything else to be under it because the ‘Exclusive’ flag is set. Chrome is the only browser that uses this flag in this way as it forces the linear list pattern. This ‘fork’ is the way WebPageTest displays changes to the list over time.

The dependency graph gives you some insight into how a browser handles the downloading of page resources. It is a fantastic learning tool, especially if you want to see the different strategies each of the browsers uses.


So there you have it, a brain dump into a blog post about some of the aspects of WebPageTest that I’ve found a little mysterious. As this has been a learning exercise for me too, if there is anything I have misinterpreted, please do tweet me and let me know!

If you would like to learn a lot more about WebPageTest, I highly recommend the book: “Using WebPageTest” by Rick Viscomi, Andy Davies, Marcel Duran.

Post changelog:

  • 11/10/19: Added waterfall for ‘Firefox enhanced tracking protection’ scenario.
  • 12/10/19: Added ‘service worker precaching’ scenario.
  • 13/10/19: Added ‘Chrome Stair-Step’ scenario.
  • 14/10/19: Added the dns-prefetch scenario.
  • 15/10/19: Added note about OCSP stapling and EV certificates (thanks Ryan Townsend for flagging).
  • 16/10/19: Added information about download chunks (thanks Pat Meenan for raising and checking). Clarified the error code presentation and added a “Without OCSP” waterfall chart for comparison (thanks Andy Davies).
  • 17/10/19: Added the ‘HTML early flush’ scenario.
  • 18/10/19: Added prefetch scenario.
  • 20/10/19: Added prerender scenario (thanks Simon Hearne & Ryan Townsend for the input).
  • 24/10/19: Added new section called ‘Hidden Gems’, including ‘How the filmstrip view and waterfall chart are related’, ‘How to generate customised waterfall chart images’, and ‘How to add custom marks using the User Timing API’ (Thanks again to Pat Meenan for clarification with this).
  • 08/12/19: Added the Long DOM content loaded (DCL) time scenario. Originally written for my article ‘Reading a WebPageTest Waterfall Chart’ on the Web Performance Calendar 2019.
  • 12/12/19: Added the never-ending waterfall scenario (thanks to Boris Schapira for the example).
  • 20/12/19: Added a table of contents for easier navigation.
  • 21/12/19: Added ‘Rogue image downloads (HTTP/2 with SRI)’, ‘Requests with no response body’ (thanks Joseph Scott), and ‘Third party blocking JavaScript’ (thanks Mike Herchel).
  • 03/01/20: Added information about good and bad HTTP/2 prioritisation (thanks Barry Pollard and Šime Vidas for feedback), and ‘How to display the bandwidth graph on mobile devices’ hidden gem (Thanks Pat Meenan & Barry Pollard).
  • 11/01/20: Added the ‘Large Time to First Byte (TTFB)’ scenario.
  • 12/01/20: Added the ‘Inactivity after SSL negotiation’ scenario.
  • 13/01/20: Added the ‘Missing component times (DNS, Connect, TLS)’ scenario (thanks again Pat Meenan for his input), and ‘The impact of a CPU bottleneck’ scenario.
  • 30/01/20: Added ‘Consequences of a 301 redirect when changing a URL’ and ‘The missing intermediate certificate’ scenarios. Thanks again to Barry Pollard for the test and information about the latter.
  • 22/06/20: Added information about the filmstrip borders, as layout shift is now highlighted (thanks Pat Meenan & Rick Viscomi).
  • 12/08/2020: Added ‘How to test a 404 page’ hidden gem. Added the ‘Vary unusual waterfall’ scenario along with a link to my blog post with a more detailed examination.
  • 13/08/20: Added information ‘Inline script execution’ visualisation, and ‘How to delve into the download chunk data’ (thanks to Radu Micu & Pat Meenan for fleshing out the details).
  • 05/09/20: Added ‘Identifying requests not from the main page document’ section.
  • 22/09/20: Added ‘How to view a sites HTTP/2 dependency graph’ (Thanks Pat Meenan for clarification on the finer points), added ‘Examining undefined 204 status codes’ scenario, and ‘Service Worker precache slowing a page load’ (thanks Andy Davies). Update: Added tip from Jeff Posnick on how to avoid the double cache download when using Workbox.
  • 03/10/2020: Added explanation of the difference between ‘First Byte’ in the summary table, and the ‘Time to First Byte’ metric listed in a HTML request details panel. Thanks go to George Liu, for the question and Barry Pollard and Pat Meenan for the explanation.