Wednesday, February 19, 2014

Some numbers on Mobile Safari rendering performance

Update 2014-04-24: I've created a very-simple-to-use OS-X app that helps you gather rendering numbers like these from iDevices. See the Mobile Safari Rendering Profiler.

In the last post I presented an overview of measuring and improving response times with Mobile Safari. I alluded to some actual performance numbers - here you have one set, taken from a heavy view switch with AngularJS.

Remember: changing the browser UI happens in 5 steps: DOM manipulation (javascript, HTML), Style calculation, Layout and Paint. Only after all of them have happened will the user see the new thing, so it's their combination that you have to keep an eye on.

Total time to render

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So an iPhone5 can render the view at roughly 3 times the time it takes latest Chrome on my mid-2010 mbp. Which isn't actually that bad - it can easily be possible to make a site that works well on desktop perform reasonably on a recent iPhone. Older iPhones are clearly a different matter.

All of the numbers I present are the average of 5 runs, with 2 burn-in runs before that. The standard deviation is was between 10 and 20% - you are well advised to get enough data if trying for micro-optimizations. There is no layout trashing in the test render sequence.

DOM manipulation

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Somewhat surprisingly, the actual DOM manipulation (as done via AngularJS) is only twice as slow on the iPhone5. This isn't just a javascript benchmark, as it includes javascript, HTML parsing and DOM API calls. AngularJS is reasonably efficient in the way it generates the DOM, though there's quite a bit of javascript overhead for the two-way data binding. Again, the older phones are much worse.

Style (re)calculation

Click on the image to see more of the data

Here we see another surprise: Desktop Chrome is 10 times faster than Mobile Safari on iPhone5. I think there's been an algorithmic improvement in Chrome compared to the now somewhat old version of Webkit in iOS6. There are no synchronous style calculations in the traces.


Click on the image to see more of the data

Roughly as expected, iPhone layout takes 2.5 times as long as Desktop Chrome's.


I'm not going to bother showing the numbers. The paint times where only about 5% of the totals. The phone typically paints a lot fewer pixels so it performs reasonably well. I think that you'll only have to worry about paint performance in animations or if you are stacking a lot of hw-accelerated layers and changing their contents so that you have to paint all those textures.

Some parting thoughts

Mobile Safari can perform surprisingly well compared to a slightly older laptop, running at 1/3rd of the speed. Although javascript performance is important, even with AngularJS we are spending about half of the time in going from the new DOM to having something new on the screen.