I guess that it is fast because modern browsers are fast, implement good algorithms to repaint just what need to be repainted, and delegate a lot of work to the GPU. Exactly like the Quartz compositor engine.
Also the real computer has to think about it more. (E.g. initialize whatever services the applications uses, be concerned about the data structures the applications mirror.). The demo just has to look right on initial inspection.
But he said "Faster". Not "Fast"
Animation speed is just an arbitrary constant.
Except when the system can't handle the animations and they get jerky.