Multi-threaded thumbnail generation?
I'm simply guessing here, because I recall gthumb being as slow as Nautilus for thumbnailing, but is GThumb using multithreading at all to generate thumbnails? If not, there would be some very significant performance gains to be had. See nautilus#856 (comment 1087259) that links to nautilus!660 (closed) for a potential implementation being worked on in Nautilus...