Canvas may be pretty well optimized by now, but it's far from perfect. When I was using it a few months ago, I needed to draw into a temporary context then draw the result into the main canvas, because of unusual composite operations. This worked well on most browsers, but Safari incurred a massive slowdown which, according to Instruments, was largely caused by time spent in memcpy and some sort of surface locking. I'm not sure why memcpying relatively small images on a powerful CPU was using so much time, but a more performant implementation would use render-to-texture and not do a round trip through the CPU, and whatever their implementations other browsers had no such problems. It was slow enough - just drawing a few 300x300 or so images at 60fps! - that on x86 it might well have ended up faster to just construct the frame in JS using array buffers; unfortunately, I was also trying to target Safari for iOS.

Considering that Canvas also has hard limits on what sort of effects you can implement with it, I hope WebGL is supported in Safari and IE soon.