I'm investigating performance issues with Matteo Spinnelli's iScroll component on "Windows Phone 8". That lead me to try out IE11 preview and the new F12 dev tools in order to get more insight into what is happening.
In fact, the same high CPU usage can be noticed in IE11 preview and from what I can tell it is caused by a layout pass directly after an elements style has been updated. But we are not changing properties that should affect layout in any way, only "transform" and "transition" properties are changed.
But once this layout is done, the animation seem to be composed independently of the UI thread (on the GPU). It doesn't sound like a problem but when you are scrolling slowly the transform property will be frequently updated and CPU usage is high. On the desktop you won't notice it but on the phone it's sluggish.
This is the demo I've been profiling: http://lab.cubiq.org/iscroll5/demos/simple/ (adding more list items highlights the problem as the layout pass takes longer).
Chrome does not seem to do an extra layout, any idea why IE need it?