Well, I was told that it's X11's fault and that the low power mode of the GPU is enough for Windows Aero, but not enough for X11. The GPU can't keep up with the memory bandwidth requirements of X11 (yes, that means that compositing on X11 is a resource hog compared to Windows, which is supposed to be light-weight.) Wayland is supposed to fix that.
The result is framebuffer tearing (which in the end is just as visible as screen tearing.) Switching the GPU to higher speed gets rid of that tearing.