I have been running Lion on an older 2006 iMac for a while. This machine has two monitors - the primary 20" and an external 1600x1200 connected via mini-DVI connector. Thus far Lion has been pretty slow compared to Snow Leopard. I thought it could be the extra pixels slowing it down on such an older machine, so I disconnected the external monitor. I was surprised to see how the performance improved somewhat. The window manager seems to be much more responsive, and I am not waiting as much. It could be a result of me not running as many applications since I only have one screen, however I can definitely tell a difference with my primary applications open. Curious if anyone else has observed this. This machine is one of the first iMac that Lion supports, so it very well could be over-taxing the video card.