I believe I’ve fixed the MASSIVE slowdown I experienced when moving from Lightroom 2 to Lightroom 3. There aren’t too many settings to tweak, but I was finding that everything was lagging - opening a new folder would sometimes take minutes, and some operations would just hang completely. But then I realized that there was a setting I’d started using to automatically apply the default develop presets, and I began to suspect that this was causing Lightroom to do a lot of resource-intensive background processing. I configured my preferred defaults as a develop preset so I could get back to them, and reverted the default develop settings to the factory preset. Everything got much faster immediately! We’ll see if it holds.
Still, I wish there was a way to force Lightroom to take more RAM.