Interesting how JS still doesn't live up to AS3. Everyone always discredits Flash... but if you're trying to make a Physics-based browser game, it still seems appropriate!
Well, the asm.js version of the benchmark is JS, and it outperformed AS3. Although as cpleppert said, perhaps an alchemy version for Flash would be faster than AS3, that would be interesting to see.
But, I'm not aware of an alchemy version of Box2D for Flash that is convenient to use from normal Flash (what presumably the rest of the game is written in), while for asm.js there is such a version (that is easily usable from normal JS), here's a demo
You could get even better performance if you directly coded against the alchemy VM instructions or used alchemy directly. This would be comparable to asm.js or coding Java against the unsafe memory operators.
