I have been using a Dell 23-inch monitor since I got my MBP about ten days ago, and I finally packed it away in the closet because it seemed to be causing more problems than it was solving. I was connecting the monitor to my MBP via a DVI to MDP cable, and I did not observe any performance issues in normal use, even when playing Diablo 3 at 1920x1080. What I did notice seemed unrelated, but in retrospect, probably was. I was using an Apple wireless keyboard and a Logitech Performance MX mouse along with the external monitor, and found I was experiencing significant lag with the keyboard, where entire sentences would be delayed, and in some cases letters were skipped entirely. With the mouse, I experienced significant stuttering, particularly while gaming. I switched to the Magic Mouse, it was even worse. The list of things I tried to resolve the issue is very long, derived mostly from Apple Discussions. Remove and re-add bluetooth devices, set them as favorites, delete prefs, change speed, install Magic Prefs, PRAM/SMC, etc... Nothing worked, so I disconnected the monitor, and all my issues disappeared. Is there some undocumented problem with external monitors and Apple wireless keyboards and Magic Mice? TLDR: External monitor + Apple wireless keyboard and Magic Mouse = lag. Did I miss a simple fix?