Performance Audit Of Web Applications By Professionals
Case Study

PerfAudit: CanIUse.com

Hello folks! This is Perfaudit’s first case study for which we chose caniuse.com. If you want to see us live auditing the website, checkout our video:

Network profiling

Google pagespeed issues

As we see, caniuse.com has a google pagespeed score of 84 which is pretty descent. There are few issues pointed out by pagespeed which are mainly due to 3rd party files not leveraging browser caching or coming unminified and uncompressed. These are mostly harmless but still can be fixed by caching these external files on the website itself so as to get full control over them.

After load profiling

Issue: Janky scrolling

On caniuse.com homepage, if you scroll down the page hovering over the list of links, you will experience occasional jerks. That is what we call a “Jank”. Lets try debugging the issue using Google Chrome’s devtool timeline. Running a quick timeline recording, we get the following graph:

Paint janks

This shows that we are shooting our limit due to some paint events. Paint events happen when something needs to be drawn on the page. This means when we are scrolling while hovering over the list, something is being drawn. Yes, its those links changing their color when hovered on. If you were to inspect one of those links and enable the :hover pseudo class on them, you’ll see the culprit style:

Jank causing CSS

If you disable the color property on :hover and rerun a timeline recording, you’ll notice that there are no more paint events crossing the 60fps bar. Better performance.

To solve this, there is a neat trick by Ryan Seddon to listen for scroll event and disable pointer events on an element, say <body>, so that mouse dependent paints do not happen:

// JS
var timeout, pointerEventsDisabled;
window.addEventListener('scroll', function () {
	if (!pointerEventsDisabled) {
		pointerEventsDisabled = true;
	timeout = setTimeout(function () {
		pointerEventsDisabled = false;
	}, 500);
// CSS
.no-pointer-events * {
	pointer-events: none !important;

This will essentially disable the pointer events as soon as the scroll starts and enable them half a second after the last scroll event fires.

There is a similar issue on their results page too. If you search something, say transform, and scroll the shown results page you will notice frequent janks. Lets run a recording in devtools timeline for this page. This is what we got:

Result page timeline

Notice those green coloured outburst at regular intervals. Those are heavy paint events causing the visible Jank! Again, if you were to use the above mentioned technique you would get a much smoother experience during the scroll.

That is it for this audit. If you want us to audit any specific website, do let us know in the comments or on twitter. We’ll definitely try to take it up as our next case study.

Do not forget to follow us for more such perfy performance audits.



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