Well, for me it isn’t and I also have a rMBP. I believe you, it’s just that you are likely looking at a software issue. (No doubt that this software issue is likely exacerbated by the requirements of the screen and it sucks either way, but in principle the graphics hardware is up to the task.)
If I close Chrome, Sublime Text and Terminal it gets close to normal speed. Exposé seems to scale very badly with the number of apps running. It's also possible that some of these apps are badly optimized redrawing themselves in some way that slows down Exposé.
