My computer speccs:
Processor
Intel(R) Core(TM) i5 CPU M 430 @ 2.27GHz (4 CPUs), ~2.3GHz
Memory:
4096MB RAM (8GB max)
Video Card:
NVIDIA GeForce GT 330M
Operating System:
Windows 7 Home Premium 64-bit

I'm 5boxing on this rig with graphic set to MAX, 2 monitors. One monitor for main character and one for the slaves.

Task Manager SS



I haven't had any major fps loss yet. I've been to Stormwind, Orgrimmar and Ironforge with main and all of the 4 slaves without fps below 60.

Programs I usually run at the same time:
Xfire
Steam
Chrome
Spotify


With your computer I don't see why you should have that huge fps loss O.o