this http://blog.j15r.com/blog/2011/12/15/Box2D_as_a_Measure_of_Runtime_Performance http://blog.j15r.com/blog/2011/12/15/Box2D_as_a_Measure_of_R... is a bit old

this http://blog.j15r.com/blog/2011/12/15/Box2D_as_a_Measure_of_Runtime_Performance http://blog.j15r.com/blog/2011/12/15/Box2D_as_a_Measure_of_R... is a bit old, but it suggests that NaCL is pretty close to native (only 20-30% slower) and jre is 2-3 times slower.