Rules are sorted in order of impact upon score
Optimizing rules at the top of the list can greatly improve your overall score.
Not every recommendation will apply to your page
The recommendations are meant to be generic, best practices; some
things will be out of your control (eg. external resources) or may not
apply to your page.
Read our how to guides and optimization explained articles for additional direction on improving your page performance.
Want an expert to help you with optimization? We can recommend partners to further assist you with optimizing your site. Contact us and we'll point you in the right direction.
Rules are sorted in order of impact upon score
Optimizing rules at the top of the list can greatly improve your overall score.
Not every recommendation will apply to your page
The recommendations are meant to be generic, best practices; some
things will be out of your control (eg. external resources) or may not
apply to your page.
Read our how to guides and optimization explained articles for additional direction on improving your page performance.
Want an expert to help you with optimization? We can recommend partners to further assist you with optimizing your site. Contact us and we'll point you in the right direction.
A request-by-request visualization of the page load. Learn how to read a waterfall chart.
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Tip: rotating your device into landscape mode may give you the necessary resolution to view the waterfall.
This is the time spent redirecting URLs before the final HTML page is loaded. Common redirects include:
Some sites may even perform a chain of multiple redirects (eg. non-www to www, then to a secure URL). This timing is the total of all this time that's spent redirecting, or 0 if no redirects occurred.
In the Waterfall Chart, Redirect duration consists of the time from the beginning of the test until just before we start the request of the final HTML page (when we receive the first 200 OK response).
During this time, the browser screen is blank! Ensure that this duration is kept to short by minimizing your redirects.
Once any redirects have completed, Connection duration is measured. This is the time spent connecting to the server to make the request to the page.
Technically speaking, this duration is a combination of the blocked time, DNS time, connect time and sending time of the request (rather than just connect time). We've combined those components into a single Connection duration to simplify things (as most of these times are usually small).
In the Waterfall Chart, Connection duration consists of everything up to and including the "Sending" time in the final HTML page request (the first 200 OK response).
During this time, the browser screen is still blank! Various causes could contribute to this, including a slow/problematic connection between the test server and site or slow response times from the site.
Once the connection is complete and the request is made, the server needs to generate a response for the page. The time it takes to generate the response is known as the Backend duration.
In the Waterfall Chart, Backend duration consists of purple waiting time in the page request.
There are a number of reasons why Backend duration could be slow. We cover this is our "Why is my page slow" article.
Time to First Byte (TTFB) is the total amount of time spent to receive the first byte of the response once it has been requested. It is the sum of "Redirect duration" + "Connection duration" + "Backend duration". This metric is one of the key indicators of web performance.
In the Waterfall Chart, it is calculated at the start of the test until just before receiving on the page request and represented by the orange line.
Some ways to improve the TTFB include: optimizing application code, implementing caching, fine-tuning your web server configuration, or upgrading server hardware.
First paint time is the first point at which the browser does any sort of rendering on the page. Depending on the structure of the page, this first paint could just be displaying the background colour (including white), or it could be a majority of the page being rendered.
In the Waterfall Chart, it is represented by the green line.
This timing is of significance because until this point, the browser will have only shown a blank page and this change gives the user an indication that the page is loading. However, we don't know how much of the page was rendered with this paint, so having a early first paint doesn't necessarily indicate a fast loading page.
If the browser does not perform a paint (ie. the html results in an blank page), then the paint timings may be missing.
First Contentful Paint is triggered when any content is painted - i.e. something defined in the DOM (Document Object Model). This could be text, an image or canvas render.
This timing aims to be more representative of your user's experience, as it flags when actual content has been loaded in the page, and not just any change - but it may often be the same time as First Paint.
Because the focus is on content, the idea is that this metric gives you an idea of when your user receives consumable information (text, visuals, etc) - much more useful for performance assessment than when a background has changed or a style has been applied.
If the browser does not perform a paint (ie. the html results in an blank page), then the paint timings may be missing.
DOM interactive time is the point at which the browser has finished loading and parsing HTML, and the DOM (Document Object Model) has been built. The DOM is how the browser internally structures the HTML so that it can render it.
DOM interactive time isn't marked in the Waterfall Chart as it's usually very close in timing to DOM content loaded.
DOM content loaded time (DOM loaded or DOM ready for short) is the point at which the DOM is ready (ie. DOM interactive) and there are no stylesheets blocking JavaScript execution.
If there are no stylesheets blocking JavaScript execution and there is no parser blocking JavaScript, then this will be the same as DOM interactive time.
In the Waterfall Chart, it is represented by the blue line.
The time in brackets is the time spent executing JavaScript triggered by the DOM content loaded event. Many JavaScript frameworks use this event as a starting point to begin execution of their code.
Since this event is often used by JavaScript as the starting point and delays in this event mean delays in rendering, it's important to make sure that style and script order is optimized and that parsing of JavaScript is deferred.
Onload time occurs when the processing of the page is complete and all the resources on the page (images, CSS, etc.) have finished downloading. This is also the same time that DOM complete occurs and the JavaScript window.onload event fires.
Note that there may be JavaScript that initiates subsequent requests for more resources, hence the reason why Fully loaded timing is preferred.
In the Waterfall Chart, it is represented by the red line.
The time in brackets is the time spent executing JavaScript triggered by the Onload event.
Note that Onload time was the previous default for when to stop the test prior to Feburary 8th, 2017.
Logged in users can record a video of this page loading and pinpoint exactly where bottlenecks and stoppages occur.
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Jump to major milestones like First Paint and Onload
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