Please try LR 4.1 RC 2.
heard RC2 has a lot of problems, dear not to try.
BTW, does LR4 using GPU?
Does not use GPU for image rendering.
LR 4.1 RC2 is here pretty slow. I guess even slower as 4.0. Most on the time, I process 10 to 22 MP pictures.
My system: Mac OS X 10.7.3, i5-2500, 8Go RAM. On a such system, LR should be very responsive. Despite LR being a wonderful software. Adobe use to deliver Beta-quality software at start (revision x.0, x.1). Most of the time we (customers) have to wait until x.3 to get a production quality software.
Adobe please be a bit more careful.
Have a nice day