So I ended up having to disable thumbnailing for Photoshop PSD files because every so often, the thumbnail thread would just suck up tons of CPU.
I realized that the thumbnails were too small at 48x48, so I moved to 64x64 thumbnails.
I also added a "loading" icon so that I can see which pictures are currently being thumbnailed. Look! It's processing them in parallel! :)
I'm starting to really dogfood this app now. Which is, of course, the whole point. I'm at 426 gigs of photos and all but 56 gigs have been backed up at least once.