My game is quite complex with lots of massive sprites, physics and 720p background with transparency (pretty much everything against good "mobile practice") and it runs flawless on devices with tegra 3 onwards, which I consider fine, because tegra 3 is pretty weak on the CPU side. Compiled through CocoonJS. It works much better than I had expected, which is me being very grateful for Scirra and Ludei.
I've tested it on a Samsung S3 class device or newer phones run it at 60 fps.
Personally I think its fantastic that HTML5 gaming is so viable now on mobile devices. As we progress towards newer devices, it won't be an issue anymore with low performance.
Also, I had the opportunity to test the html5 directly through a Nexus 7, and it ran at 60 fps before compiling. So the future looks very bright for html5 gaming. Mobile advances really quick, hardware seem to be twice as fast every year.
Edit: Ofc I had to sacrifice a lot of visuals to get it to run smooth on older devices, in my own experience, WebGL effects in construct 2 destroys performance utterly. So do not use it everywhere. An example, I had a scenario where my fireflies used the effect GlowHorizontal, and with only 10 of them on screen FPS was cut in half on a tegra 3, becoming unplayable. So i compromised and used additive blending with some transparency sprites and it maintains a "glow" look without any performance hit.
Edit2: One of the reasons I have not been impressed with Intel's new Crosswalk XDK is the inclusion of ~20MB of overhead into the APK. Mobile devices can chew through high resolution texture sprites easily these days, but with the 50MB limit on app size, it's a SIN to have 40% of the app being XDK overhead.