Ya, dual processor, and a good video card
Hmm, perhaps massive amounts of mining could lag as well, since it has to create the mineral chunks, like I have played a map where the players couldnt mine due to ccmu.
Hmm, I played a UMS not too long ago. The creator obviously never even started it in Starcraft. I was able to build like 10 more drones and 6 zerglings, later I managed to add 3 lurkers. I couldn't press any more larvae out of my hatcheries from there on.
My drones were mining, but quite often it wouldn't draw the mineral chunk sprite. It still delivered the (invisible) minerals, though.