I've been using SketchUp since version 3, though mostly for fairly lightweight projects until now, but I haven't used V6 until recently. I've got a simple file with 4 book cases in it. It's about 1.3MB, and it takes SketchUp sometimes 10 seconds to save the file even after a trivial change.

MacBook Pro, 2.16 GHz C2D, 2GB RAM, 25GB of free space on my hard drive and, perhaps the thing causing the problem: 10.5.1 -- anyone else seeing some slow-downs post Leopard install/upgrade? (For the record, I did an "Archive and Install.")