I think if this were me and were talking windows machines here I think then I would go get the SysInternals suite and use its process viewer to see whats happening on the PC when its running. That version is better than the built int ctrl-alt-del process viewer by a long way.

It sounds like there is a resourcing issue whether real or fake. It would be useful to know whether its CPU bound or memory bound or whether its paging stuff out when its going slow. You can get at all these details using the viewer. You need to get the PFDelta (page file delta) thats how many pages it did in the last second or whatever time slice your doing. You need the overall RAM used (the Working Set) and the CPU time is always given as a default. Are you able to show some of these values when it near hangs. The PFDelta is likely to be showing the problem tho.

You might also check if your using an anti-virus. Maybe unplug from LAN and switch it off temporarily and see if that helps. For apps which constantly save files to HDD then the AV has to check them continuously and that can be done at the kernel level (because root kits can override the check) which involved more hoop jumping and slowdown.

Still, could be a stright up Gimp programming issue tho but best to check whats going on first.