All it did was blow up my browser's RAM usage to 2GB and then deliver ~2fps.
IIRC Flash did better.
If by "better" you mean "blew up my browser's RAM usage to 2GB AND maxed out an entire core" then that's consistent with my flash experience (on 64-bit Linux, which I realise is not exactly a first-class citizen)
I agree complaints like this are currently valid in some respects, but the good thing is that feedback loops like this will eventually result in the same efficiency we come to expect from flash games on HTML5.