Performances are affected by several factors:
1. data size of your model
2. number of links: this is very true at loading time. A spaghetti plate of external links dramatically slow the loading of your model and any modification (update graph is complex then!!)
The problem is mostly...
GSMExtract points to a BRep feature whereas GSMBoundary points to the spec itself, that's why you are more liable to have unstability or problem at replace.