I'm running 6 clients stacked @ 1920x1200. i7 2600K @ 3.4 GHz, 16GB RAM, Win 7 64, Radeon 6990.

I run my main at 30FPS and slaves at 15FPS and get lower frame rates at times. Main is on High Quality, slaves are on the minimum for everything. Make sure shadows and volumetric clouds are off for everyone.

Under Settings, Performance make sure Hardware Occlusion is Off and the next 4 are On.

If you suspect a bottleneck (either CPU or GPU) here is how to find out.