High framerates on background screens will really hamper your ability to play smoothly. The more intense the graphics, the worse it will be. For WoW, 15-25 on slave clients is more than enough to look smooth and not brutalize your main. For EQ2, anything higher than 13 FPS on the slaves gave me a huge hit in performance.

Your slaves do not need huge framerates, you'll barely be able to see the difference but you will feel the difference. Try dropping your slave to 15-20 FPS and make sure it is using lower quality graphics settings.