Hi,
I too experience a slowdown in response from LR4. I use a MacPro with 5 GB of ram (planning to upgrade that at some point soon). As I use LR, with a only few other programs such as Safari, ... open, I note that the free ram eventually gets down to only a few MB's. I wonder if somehow LR keeps eating up ram and not releasing it. A restart restores the available ram; an inconvenience but in my environment it is not too bad. But LR4 feels slower than LR3, but the tools are so much like working in a traditional darkroom!!!
Jean-Michel