I use gimp 2.6 under gnome on linux, I originally rebuilt my own 2.6 as the 2.8 default was very slow. *but* having built 2.6 I found it just as slow, ultimatly the real reason for the slowness under gnome3 was there equivalent of compositing enabled dsktop is outrageuosly slow and innefficient.
I revertd linux back to ggnome fallback mode and speed was restored, however I prefered 2.6 so I stuck with it.
As often happens when prgrams reach maturity, changes start to get made for the sake of change and whilst it's always a matter of taste often the baby get thrown out with the bath water.
2.4 -> 2.6 big improvement (for my taste)
2.6 ->2.8 step backwards (for my taste)
BTW if speed is an issue SketchBlock is faster than any version of GIMP