World of Warcraft has been running frustratingly slowly on my Retina MacBook Pro. Though it has a lot of pixels to push the machine's CPU and GPU are quite fast… and Warcraft had been running much better.
In fact, it was now generating around 8 frames per second (type control-R while running it to get the fps displayed). I tried exiting other applications, killing background software, rebooting, nothing helped. Then it mysteriously improved and was quite playable for a while (30fps). A couple of days later for no apparent reason it cranked back down to 8fps.
It turns out that a classic "mysterious Mac problem fix" actually fixes this problem: resetting the SMC. The SMC is a subsystem of the Mac which helps manage low level things like power and heat. My best guess is that at some point the GPU got too hot, the SMC throttled its performance to try to control the temperature, and then got stuck and forgot to unthrottle it.
On a Mac with a non-removable battery you can reset the SMC by shutting down the Mac and holding down left control-option-shift and the power button all at once, then releasing them all all at once. Then turn the computer back on and the SMC should have been reset. You can read Apple's support note on resetting the SMC if you want more info.
Original info from this thread on Apple's web support forums.