Did you increase the shared memory limit? I am not sure what happens if it the X server hits the limit but I guess it just silently stops allocating more shared memory.
Err, I know somewhat how to mess with POSIX SHM in applications but how can I change the shared memory limits?
From my experience GIMP-2.0 runs very nicely on MacOS X. I even had PS users claim that the GIMP-2.0 user interface would feel more responsive than PS on the same machine.
Description: This is a digitally signed message part