In that case, as most of the commenters on the thread say, ranged DPS is probably your best bet. You won't be able to pour out as much aggro as someone who isn't lagging, but at least you'll be able to use your 3 frames per second to do some damage, and you're sure you won't draw aggro and wipe the raid. As for which ranged DPS class, though, I'm not sure -- Hunters have autofire, so you can be sure that you'll be pushing out DPS constantly. Mages aren't too hard to raid with (most Mages will actually tell you that they just press one button over and over again). Warlocks might be easier with their DoTs, but even that relies on timing in a way that Hunter DPS really doesn't. I'd say Hunter.
Some people are saying Healers, which I originally thought was a bad idea, but if you combine their recommendation of just looking at the floor (to speed up the FPS) and watching the healing meters, you might have something there. Wouldn't be very fun (whack-a-mole FTW), but you'd be helpful to the raid in situations where you wouldn't have to move around much. So healer might be a viable choice for a low-end computer user as well.