A couple months ago, I bought two fairly beefy desktop computers, and shortly after getting them I started loving making kitbashes again. It was soooo much faster than my old computer. No more running out of memory!
Well, at some point, I installed GIMP 2.8 and I didn't really notice it right away, but really the performance was terrible. I was bummed out because my desktop at work, which is beefy, but not nearly as much as my home machine, was much faster at certain operations (like scrolling, or extending selection regions - REALLY common things like that). I was kind of just living with it up until last night, when my machine blue-screened. I'm a little worried about what this might mean for my hardware, but everything else runs great on my computer, so I did a little searching with Uncle Google.
It turns out, in GIMP 2.8 there's a section under Edit > Preferences for Color Management. Setting the "Mode of operation" to "No color management" made all the difference in the world for performance. There is absolutely no lag in any operations now. I don't know why my machine has issues with color management, and maybe when I have some down time I'll look into it. I'm hoping it's some driver issue that also tickled whatever caused the blue screen.
Anyway, if you're using GIMP 2.8 and are having performance problems where things like making selections or scrolling are really slow, try turning off color management. It just might help.