are you doing this at the part level or editing a part in a assembly???
SW 2012 has a Feature Freeze function that will freeze rebuilds at a specific point in you FM so your don't have to wait for unnecessary rebuilds.
Nobody here knows what SW 2013 will do. Well, there might be some who know, but they can't talk about it.
I've noticed that SW uses multiple cores more when I am doing surfacing work than solid modeling. Since each individual surface is a body, I suspect that multi-body parts may take advantage of multiple cores better than single body parts, but I have never done any testing to prove it. If that were true, it might make sense to build complex parts as multiple bodies and then merge them only at the end. Actually, it might make sense just to minimize the number of features to be rebuilt at any one time.