When I installed Lightroom V4 I noticed a big performance hit in the Develop Module. When I would move the Exposure slider I would see a second or two delay before the change was visible on the screen. I also noticed that the Spot Removal brush would take two or three seconds before I would see the results. In Lightroom Version 3 I had none of these problems.
Lightroom V4.1 improved the performance a little bit, but still didn't get it back to the level of Version 3.
THE SOLUTION: Turn OFF (set to zero) the Luminance Noise Reduction in the Detail tab. When you are done with your editing for ALL your images just change the Luminance setting to your preference and sync it to all the other photos.
THE OTHER SOLUTION: Set the Process Version to 2010 under the Camera Calibration tab, but the problem with this is that you no longer get the benefits of the 2012 Process Version.
FYI: I have an eight processor, 2007 vintage Mac Pro with 11GB of Ram.