Im running into a similar problem I have a Gf8600. If I run all 5 accounts on the same monitor everything is fine and playable, even in Dalaran. Outside dalaran im anywhere from 30-60fps on all 5 windows. Dalaran it drops to 10-20ish.

Screen real estate is small, so I attached a second monitor to put the 4 slave accounts on and WAM! Everything drops to 5fps. Maybe 15 if im lucky. And it does this even if I have just 2 accounts going. One on each screen. I put in a second video card. A Gf7900, hooked the second monitor to it, and it did the exact same thing, so I dont beleive its a hardware bottleneck.

I havent tested it yet, but the only advice I was able to find to fix this was to set the displays into a 'stretch' mode. or extended desktop useing the Nvidia control pannel. Not the windows display controler. Its the goofy one where if you maximize a window it will streatch across both displays so its acting like one large one. Main reason I havent tried it, is that my monitors use different resolutions and that mode forces both to the same rez so it looked all funny, not useing the native resolution.

It might solve your issue.