I’ve been testing Premiere at the office with the idea of switching from FCP. Our main systems were Snow Leopard at the time I started and the performance was problematic. It wasn’t nearly as bad as you describe, but I have a 12 core mac with 32 gb of RAM. Our other octo Mac with 8 gb didn’t work out so well. The program lagged a lot, especially in the interface/timeline, dragging clips around, etc.
I upgraded our computers to Lion and Mountain Lion and the performance on both computers got instantly better. Realtime performance is great and only minor complaints. Still haven’t fully switched over, but the OS upgrade definitely helped.