The bone behavior doesn't do anything CPU intensive at all, as far as I know. It just does a bunch of interpolation functions which are really really cheap for a CPU to do. Have you got some framerate figures that demonstrate slowdowns are caused by the bone behavior and not by other things like effects on the sprites the bone is controlling?
As I said in another thread, the key to optimisation is to measure! What are the framerates? How much is bone slowing things down?