But accurate, its only ever faster if you do not combine. So the operational word is combine. If you combine its allways slower to not combine becasue theres more operations in the chain. This said there is a slight possibility that if you have a lot of operations after the blend that operate on all points in the object. Now if thats the case you can rig the thing 2 times once for the proxy and once for not the datas freely transfeerable so its faster on all accords. Besides most likely the history is wrong anyway.
If you combine then you will allways be doing the combine which is slower than no combine. Since maya prunes the data to only those things that need to change it doesnt really matter if you have the rest with it or not.
It can be faster if you do not combine, however theres really no trick in haveing 2 chains that produce same things side by side for the separate had only sowing approach so its still not much idea to combine after the fact.
So no its not about how you combine. Combine is much heavier than a simple blend even IF it operated on all ponints wich is does not if you instruct maya to not operate on it.
In any case i digress thsi separete head and combine thing is one of those things why programmers get told not to prematurely optimize, somebody allready did it better than what you get by hand.