Someone chime in if I'm wrong, but it is my understanding that choosing the smaller face counts in the options dialog when building the mesh doesn't actually reduce the amount of processing the system does. Selecting a lower face count is just adding a decimation step automatically at the end of processing.
Say your high face count is 10 million, and your medium is 5 million. If you select the 5 million, it will generate the entire 10 million face mesh, then decimate at the end down to the 5 million. That's why it's always best to go ahead and run the high face count for the mesh, that way you can do whatever editing you need to do, first, then smooth and decimate as you need.