Fire up the animation test in Chrome's timeline and view the frames. Replicate the same animation as a CSS transition. view in the timeline and report back. Hint: Pure CSS transitions are faster than JS driven ones. Not sure why so many people seem to think otherwise.

They also take longer to start. More importantly they're impossible to control.

Came here to say the same thing. I have done a few extensive benchmarks internally and the conclusions were exactly the same (even though you don't need benchmarks really to know there is a difference). CSS transitions/animations have less overhead than Javascript ones. I think the issue here is perceived performance vs actual performance. People assume just because it looks fast and runs fast, that it is fast, when it in-fact comes with some overhead (that might not be visible on the surface).