Left out the "compared to the MBP" part, but whatever... the Macbook external display option works fine at 1920x1200; it just works better at 1280x800."unwatchable" and "extremely laggy"?
We like to watch missed TV episodes online. On the Macbook's internal display they play as well as watching cable TV. When dragged to the 24" Dell at 1920x1200, the frame rate drops to the point of what I personally consider to be unwatchable. For some, it might be good enough. I'd rather watch it on TV. On the MBP, when dragged to the Dell it plays the same as on the internal display.
We're licensed to run a standalone visualizer similar to the iTunes visualizer at performances. On the MBP it runs smoothly at 1920x1200 even while running other applications. On the Macbook at 1920x1200, the framerate noticeably drops and it heats up fast, though it runs fair enough if that's all it's doing; at 1280x800, the frame rate is smoother and the temperature is better, so that's what we run it at.
I mean c'mon... the MBP runs Unreal Tournament III at max settings, while the Macbook gets hot running Mahjong Towers Eternity. It's like comparing apples with basketballs.
But hey, it bothers me how on the MBP the RSS Visualizer screensaver lags a bit on the external display behind the internal display. Meanwhile, I've got a friend who can stare at a 1080i HDTV right next to a large standard TV and say, "I don't really see the difference." To each his own.