Physbones are quite well optimized, and within reason can be considered pretty cheap. It seems that the frame time of PhysBones is mostly reliant on how many transforms they animate, at a rate of 2 ms per 1000 affected PhysBone transforms.
The component hierarchy shape (what is parented to what) and amount of components seem to have a slight effect on this, giving me a 33% difference between extremes, where fewer components is better.
Collider count has a very slight impact on frame time, and the other settings seem to have no noticeable effect.
Disclaimer: Since PhysBones’ runtime is very different in VRChat and Unity, these results are gotten from tests within VRChat. While usually the results are verified in VRChat, this time all the data is from VRChat, and therefore the results may be less accurate.