This bug regarding the Nautilus image thumbnailer performance was reported almost 6 years ago. It had input on it at the rate of around one message every two months, up until the end of 2003 – then nothing until 2006, where duplicate bug reports start coming in pretty regularly until the end of 2006. All pretty quiet until then, kind of suddenly, Michael Chudobiak writes a patch that speeds up Nautilus almost 3400%
I used a test folder that had four 15000x400 tif images and four 15000x400 png images (solid colors). Without the patch, it took Nautilus 4 minutes and 30 seconds to thumbnail the folder. With the patch, it took 8 seconds. I'm not clever enough to touch the actual pixop codebase. But these numbers suggest there is enormous room for improvement! This bug has been open for 6 years - nudge, nudge.
Because free software is forever we can just afford to get there in the end :)