(From simonsarris.com' blog)
Prendering paths isn’t a magic bullet, there are still a lot of uses for drawing paths constantly in canvas. If you are making a live drawing application, or otherwise constructing dynamic paths and/or moving and animating paths on the fly, then any kind of pre-rendering is going to be nearly pointless or even harmful to performance. After all, what’s the use of drawing something to a separate canvas and drawing that state back to the original canvas if the path changes constantly? You’d need to re-draw the in-memory canvas just as often, so you’d lose all benefit.
It is also worth mentioning that you might want to play around with using PNGs instead of in-memory canvases. Another thing to test is putting multiple paths onto one in-memory canvas versus putting them all in their own separate canvases, effectively making a sprite sheet. From previous tests, it seems that there is a slight advantage to giving each sprite its own png instead of using a single-png (or single in-memory canvas) sprite-sheet, but it wasn’t that big of a difference.