FPS problem while addding the more vehicles

hi,
in my project if i increase the physics vehicles from 7 to 12 my fps goes to 70 to 20,but if i comment setmassmatrix my fps will be good,so please tell me how can i increase my vehicle count with much decrease in fps
thx in advance

wabaler once go through this please

make vehicles that are not used freezed and raycast vehicles are faster i think...