Yeah, my experience roughly matches yours. In practice Firefox, IE and Chrome currently have very fast 'fast paths' for Canvas, but if you hit the slow path - some operation that isn't optimized - you can see a 5x performance hit, or a 50x performance hit. And because Canvas's feature set is so limited your only workaround is usually to do it in software. Composite operations are definitely not consistently optimized across browsers.

It's worse on mobile, where for some reason way less effort has gone into hardware accelerated canvas - there I think you may just be stuck using WebGL.