So here are the quick results. 1600 circular objects in zero gravity... It was obvious that chipmunk is the most demanding on the cpu. Regarding the other two physics engines, there is little difference between box2d and asm.js performance, IMO.
These tests were run on Chrome Version 40.0.2214.111 m using r195 and r196.2. r195 was way smoother as you can see when comparing its graphs with the number of the frequent number of large dt values that are apparent in the r196.2 fps graph....