I always seem to have to edit and mix vocals in a separate session , that takes up the most CPU for me , I guess cause there are so many plug ins going on. It sucks when vocals arent fitting , then I have to switch between sessions and eq the instrumental or whatever... I usually just bounce a wav of the latest mix into the vocal session, it seems to be the only way. And I have a quad core , Pro Tools is the only thing that ever freezes my computer.