TBH it's a small demo - I've heard while JS engines these days are fast, they still struggle with complex physics simulations. I can't see any performance details/improvements noted in this release - anyone got figures?
Unfortunately the game was taken down due to the Japan situation. Understandable.
Under Safari i was able to simulate ~80 live physics objects in a high collision environment at full frame render, bound to the DOM at 60fps on reasonable hardware 6 months ago (safari still has the fastest DOM paint time out there now).
Even when using Firefox 3.6 i could manage around 20 dynamic objects.
No CSS3 animation, just math and re-paints.
Javascript Math is not slow (at all), its generally the DOM/Canvas slowing you down.
I'm really impressed with what JS can do these days...