Please refer to this video: http://www.youtube.com/watch?v=0ua0bxi1piM
That video compares the performances of Gimp 2.6.12 and Gimp 2.8.2 on OS X 10.8.
It appears that gradients generation of Gimp 2.8 is about 2 times slower than on the 2.6 version (even when color management is off). By the way, for some reason when I did the video capture it took about 2-3 seconds to generate a gradient on a 1650x1250 canvas but usually it takes about 10 seconds. I also have a Windows partition on the same computer that has the same version of Gimp installed and these lags don't occur when I use it so it's probably not a hardware issue.
Also, even though it's not a bug, notice the inability to use the opacity sliders to change the opacity level by multiples of 10 using 2.8 even though it was very easy on 2.6. I use this feature a lot.
And finally, it's impossible to select the Normal layer blending mode if there's no space for the menu to draw on the top of the screen.
I very much appreciate the work that has been done porting the Gimp to OS X and I hope that these bugs will get fixed soon.
I see the same performance issue with the native built Gimp 2.8.2 on my 2011 macbook air with OSX 10.8. Toggling visiblity of layers that contain areas that are filled with a color or pattern takes much more time with the native built 2.8.2 than 2.6 on the same machine. It takes seconds before the shape starts to redraw and the redrawing itself is much slower.
However I have also installed the 2.8.2 version that still requires X11 and this version seems to have the same performance as the 2.6 version.
So it seems to be the native implementation on OSX that is the cause for the perfomance degradation.
For now I use the 2.8.2 with X11. Really appreciate the single-window mode!
I couldn't even get the 2.8 X11 version to work properly so I'm not sure it's a native version issue.
I loaded the native 2.8.2 for mountain lion and found that the layer toggling and curves tool are VERY slow. I was hoping the native version would be faster :(
I'm back to using the old 2.8 that works with X11, but now I don't have a few plugins that I want, such as BIMP and G'mic.
Can someone point me I the direction of some plugins that I can use and/or comment on speed issues with the native GIMP?