[azakai]

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

http://kripken.github.io/box2d.js/webgl_demo/box2d.html

So comparing currently usable versions of Box2D, the fastest JS one (the asm.js one) is faster than the fastest Flash one.