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2.8.6 running on Mountain Lion
When using Select by Color, there is a considerable performance difference between the Windows build and the Native OS X Build. I'm running on a Mac Mini i7 with 16GB of RAM; my Windows machine is the same CPU, but with less RAM. On Windows the select by color on a 4752 × 3168 image is less than a second; on the native OS X build, it takes almost 30 seconds with the same image.
I just tried 2.8.4, and this is as fast as the Windows build, so I'm only having the problem with Simone's 2.8.6 build.
I am experiencing the same issue. Running it through instruments it seems more than 70% of the time is spent on synchronization between threads instead of doing work. Setting the number of processors to use to 1 in the preferences is a potential workaround. It will then run in the amount of time you would expect.