You should freeze the physics when it has settled, and if the limb don't move, then don't update the bone. and you should make the update only if it is on the screen, the physics could run in the background but the model should not be animated if it is off screen,(only something that can be useful in real games)
It is a bit fps expensive ass it is now, but you can lie with it, but it is TO expensive for a non moving ragdoll.

(sorry for my bad English.. have no time to run my spieling though Word)