When connecting a NURBS muscle to the Muscle skin deformer, only certain points are calculated for Sticky weighting. Later on if you weight a point that was not properly bound, the deformer appears to get slower since it now calculates this information on the fly during deformation.
To fix points that have weighting to muscles that they were not originally bound to, you can use the following auto-fix option:
The mesh is checked, and any muscles/points that need it are corrected.