I believe that may just be the rather underpowered onboard GPU powering that massive resolution w/ desktop animations than Mountain Lion..
Aren't the rMBP pretty twitchy, anyway?
Windows 8 on boot camp has no issues with window rendering or speed. The 650m is a very capable card, and the HD4000 should be able to handle a lot of what you throw at it. Also, switching graphics cards manually makes no difference. It's a lot of pixels, but if it cannot handle the simple resizing of a window (drops down to about 4FPS when I resize the notes window), then it's definitely a software problem.
Heck the 650M can even run Skyrim at ultra settings (@Full HD resolution with 8xAA) at 24 FPS according to notebookcheck. If you can manage to use more resources than that when simply resizing some windows at high res, you're doing something wrong.
What I expect is happening is, they pass everything through the CPU after rendering, which bottlenecks the graphics. I seriously hope they updated graphics performance in this build, otherwise I'm seriously considering selling my rMBP, and getting an ASUS or something.