HTML5 games are still a bit too slow for me. I'm on a modern machine, but if I'm doing a heavy compile in the background it shows how slowly HTML5/canvas rendering actually is. [If it could only start using my GPU?] I'm not sure if Flash uses my GPU, but it's fast enough so that I can play a similar game while doing background compilations.
I thought Flash only uses the GPU for video rendering. Have you tried it in IE9 or one of the recent builds of FF / Chrome?
Regrettably no. I was not aware there was a FF or Chrome available for my OS (which is OS X) that takes advantage of the GPU.