I have the 21.5" iMac and it's hooked up to a 23" Dell IPS panel through DVI. The Dell display is the U2311H for those who want to know. I have the Dell configured in a vertical orientation as the 2nd display but here's what I've noticed. If I grab a window and drag it around on my iMac screen, it moves very smoothly and I can drag it pretty fast with no jumpiness in the flow. If I then take that same window and drag it over to the Dell, the window will move in short jumps instead of smoothly unless I move the window rather slowly. I'm not display guru so I'm wondering what might be causing this. Is it something with the specs of the Dell display I should have been looking for and missed?