Save low resolution, layered images of effects you think you might use later. When you go back to them you won't have to open the larger, final version just to see what you were doing. You can always go back to the saved, layered version later to make changes. Get very fast hard disks. IDE's are generally fast and getting better all the time.
SCSI disks are often even faster.
Ultra-Wide SCSI may be the fastest available. But Linux caches information about applications to speed their start up later. After Gimp exits, the memory it was using can safely be used by other applications. 3D hardware acceleration does nothing for the Gimp. Gimp is a 2D tool, even if it can produce some 3D effects.