Fef had a post, towards the start of Cataclysm (so within the last month or so), regarding video settings. His post was for running higher settings on the main computer and lower settings on the slave systems.

You can, via /slash commands, lower video settings below what the sliders in warcraft - video options will allow you to go. You can completely disable shadows (level 0), while the slider gets you a range between 1 and 5. That kind of thing.

I have my slave windows lower than the minimum settings, for everything except view distance. Definitely helps performance on the main window.