Try it in IE9. Rock solid 60fps pretty much no matter what's going on. Most browsers are compiling Javascript to machine code and can reach speeds close to C++. Browsers are also heading towards hardware-accelerated canvases - IE9 is the first to do it really well, and uses the GPU for all rendering. It doesn't really matter what programming language you use to send draw commands to the GPU (whether its C++ or Javascript) - it's always going to be blazingly fast so long as you offload all the work to the graphics card.
I think Firefox 5 might now be a lot better too, but I'm on an old laptop and the troubleshooting page says it's blacklisted my driver! Give it a shot, it's supposed to be using hardware-accelerated Direct2D (the 2D version of DirectX for Vista and up).
So shortly nothing will be slow in HTML5 - people just get the impression from old browsers which aren't optimised yet.