Please update layer thumbnails on mouse-release instead of every screen update.
Right now, performance suffers because Photoshop tries to update the thumbnail on every single refresh, while I (and I'm guessing anyone) don't need that information real-time.
I'm having a document with a *lot* of layers (Designing screens for an interface)
I'm using an iMac, latest OSX, latest CS6, 2GB ATI Radeon videocard, 12GB RAM.
I was rescaling two vector shapes (single lines) with dashes, and that behaved sluggish. They were a few layers apart. I noticed a significant increase in performance when I turned the layer thumbnails off.