The physics engine isn't the fastest right now since Javascript isn't ideal for such computationally intensive code. It should work fine for games with relatively low physics body counts, though. Also don't forget it's just one feature. Other parts of the engine such as the WebGL renderer are comparable to native engines. CocoonJS also provides a native-powered physics engine which is a lot faster (although only a subset of the physics behavior is supported...) and we've been experimenting with an asm.js compiled version of the physics engine, which allows it to perform at close to native speed even in a browser. However there are some tricky technical issues around doing that, but it's probably solvable, so we might have a solution in future.