Yer system seems fine...might wanna consider disabling noncrucial addons(or multiple dmg meters, only one char needs it) on the alts, turn down the graphical settings & resolution on the alts(or possibly up, as that's what happened with me...3x 1024x768 windows = baaaad lag, 3x 1280x1024 windows maximized to match my desktop res = juuuuust fine)

However, you should be scanning for virii and malware/spyware often regardless, wow performance killer or not it's not a good thing to have and leaves your computer available as a zombie for remote DDoSes(lolz exaggerated example) and other such crap; keep 'er clean