I have an imac at home - 21.5 mid 2010, and it is working great. I recently brought home a computer from work - a 24" 2009, which has been rock solid and responsive ever since I got it.
But now that the two of them are in the same space - less than 20' apart, they seem to be fighting, slowing down the 24" significantly, and the 21" somewhat too. When I first set them up, the bluetooth mice and keyboards were battling, but that seems resolved now. What else to check? They're on the same wireless (dlink) network. They don't need to be able to see each other, and neither has file sharing on.