I think that's consistent with it being singlethreaded. AIUI, unless you pin the process to a single core, scheduling can make it bounce around between cores, and the probability you'll land on a different core (and, thus, incur more cache sync issues) goes up with the number of cores.

GIMP decides to optimize the plugins for each individual core in the system, so the complexity of startup increases with the number of cores present. It's a single threaded process, so more cores = takes longer. At least for the first startup when that happens anyway.