CASpyr:
I 'upgraded' to Lr4 RC2 over the weekend. I think I'm seeing the same thing you have complained about - severe degradation of the responsiveness in the Develop module. With just one adjustment brush added, the image became so slow in responding to clicks, I couldn't tell where I was anymore. Re-draw and re-size are particularly bad.
I shut down the system and rebooted it, which seemed to help. Interestingly, I saw a lot of this when I first loaded Lr4B but it settled down after a while and was not so aggravating. (Or did I just get accustomed to it? I've never heard of a 'break-in period' for software
) At any rate, I'll leave it alone for now and hope that they get it fixed in the final 4.2 release.
If it does not settle down, I may be back to ask how to roll back to the 4.1 general release.