I am using LR4 on a Mac with 8 gb ram, I7 processor and OSX.
I have been using LR since LR1. LR4 is terribly slow. Trying to move photos from one folder to another takes about 20-30 seconds per folder on file sizes of about 6 to 12 mb. Is Adobe aware of this? If they are, I'll be patient. If not, it will force me to consider changing programs.