I've heard mixed reports of UI lag and other issues relating to the GPU performance in a retina-class display. But those were primarily based on the 15" rMBP. How about the 13" rMBP? Since it does not have a discrete graphics card, are these issues even worse? I'm debating buying the 13" (15" no matter how thin/light is too much and too big for me) and another thing holding me back is the "useable space" of the screen. I'm used to 1440x900 (I think 1280x800 is too little space on a 13") resolution. I know the "best" resolution for the 13" rMBP and the one by default is a useable space of 1280x800. If I adjust that to 1440x900 or even 1680x1050, is it still more clear than my current 13" 1440x900 MBA display?