I migrated from LR3. I never installed the beta because I didnt want to deal with BETA ISSUES. I wanted a product that worked as it should immediately out of the box. [Does ADOBE understand that???]
I BOUGHT LR4, installed it on my computer, updated the library, and then opened an image I had been working on but not completed in LR3. I clicked to update it to the newer processing and IMMEDIATELY there was torrential lag. My CPU Usage meter showed maxed out. I decided to reset the image and start from scratch. This worked for a very short time. As soon as I began using the adjustment brush the lag started building - the more I used it, the worse it got.
I then decided to open an image that had never been opened in LR3. Same issue. As soon as I started using the adjustment brush or the spot removal there was lag. and it got worse and worse the more I used it. Again my CPU meter showed maxed out. My RAM never jumped about 50% utilized though....
I am running: Intel® CoreTM i7-740QM nVidia GeForce GTX 285M 1,024MB PCI-Express GDDR3 DX10 8,192MB DDR3 1333MHz Dual Channel Memory 7200rpm serial ATA drives Windows 7 64 bit OS