I have my four slaves running full resolution in 640x400 windows and barely noticed any performance decrease from running them in a lower resolution (running the slaves in a lower resolution will make them stretch when they are pip:ed to your main screen btw.)
Did you try disabling all addons? I know Bagnon is harmless (at least for me) but do you have a Cartographer module that's being nasty perhaps? QuestHelper can eat A LOT of resources if you have set its calculation performance over 100% (/qh perf.) Not tried QuestGuru so don't know about that one. Other than this the only thing I can think of is some kind of driver misconfiguration or bug, sorry. Maybe they can help you over at the nVidia forums?