You may know already, but I've noticed some heavy performance issues after I upgraded from gimp 2-6 to 2-8.  I use GIMP a lot for personal projects or for friends, however this makes it rather difficult to use.  The actual editing still functions fine, however things like the outline for brushes lags behind cursor movement making it challenging to manage precise brush work.  While that is somewhat troubling, the biggest problem here is when I use the middle-mouse click for dragging the image around something goes weird with the displaying of the image and it tears everywhere, and can take a few moments to clear up the image (it may stop responding for a second).
While doing this, I was watching my CPU usage, and it appears that gimp completely loads one of my cores while dragging the image around, it spikes to 50 for gimp-2.8.exe and stays there until it clears up the image.  There is very high CPU usage for just moving a simple circle brush over and image as well. (there is no load on graphics card)
I'm running on Windows 7 Ultimate 64-bit, using an AMD Athlon 7750 dual-core (2.7GHz)