When I used to program under windows, long time agoThere is this nifty control called NSScrollView which implements a window with scroll bars. It would store the already drawn content in a buffer and when the control is scrolled, rather then redrawing everything it will only redraw the new portion (i.e. that which wasn't visible before), and copy the part of the previous image from the buffer. For example, if you scroll half a page up, the NSScrollView will copy the bottom half of the old image which now becomes the upper part of the new image. Only the bottom part of new image has to be rendered.
So, if those optimizations are already being used, and if the scaling is offloaded to the GPU, then why the lag is still observable? What did Mac OS X engineering team miss? When I say lag, I am not only referring to scrolling heavy realtime updated content mentioned by other posters, but also to mission control stuttering...
Although I don't own a rMBP, I have been many times and spent many hours testing and playing around @local apple store... more than some fanboys who own a rMBP and use it mainly to show off and post duplicate sect craps on MR...