I never looked for benchmarks comparing native vs ExCanvas
implementation of the Canvas tag, but all I can tell you is that the
VML implementaion is about 10 times slower than the native one.
It all depend on what native implementation (Firefox, Safari, Chrome
or Opera). The ExCanvas is also very limited by the number of shapes
and lines : it will get slower and slower as long as new lines and
shapes are drawn, whereas the normal implementation will never get
slower for this reason. The only way to make it not so slow is to
clearRect() regularly the canvas when using ExCanvas.