Has anyone been using a 13" rMBP at a 1680x1050 resolution? Are you seeing any lagginess in screen updates, scrolling, etc? Does adding an external monitor (for a dual-display setup) make a difference? I'm not worried about gaming frame rates (since I don't game), but use the hell out of my computer for work, which involves some graphics and a lot of web surfing. I would like to be able to use the laptop at a higher-than-pixel-doubled resolution, with an additional external display (also 1680x1050), without any decrease in apparent UI responsiveness.