That's a long story. In the initial releases of Falling Bodie s, our main concern is to make the difficult situations work right. Speed will come later. Some speed hints. In general, big collisions in which much energy must be dissipated in a short time run slowly.
Free fall is very fast.
More joints in the character slow the simulation down linearly. Multiple simultaneous collisions slow it down considerably. Joints up against joint limits also slow the simulation down considerably. The complexity of the scene geometry doesn't matter much up to 25,000 polygons or so. Very complex scenes may slow down the startup of Falling Bodies, but once it's running, geometric complexity usually doesn't slow things down visibly.