Yep same here. Using the RC, rendering 1:1 and all the other optimisations reccomended. The program is not as responsive as LR3.6 but as SOON as I touch Noise Reeduction or Spot Removal then the entire program grinds to a halt while it takes five minutes to make the last adjustment. That's the end of the editing session for me. It's unusable and a complete waste of money and it is clearly affecting a lot of people. Back to 3.6 and I think time to ask Adobe for a refund (and yes I can hear you laughing in the back sir).
I can confirm the issue. Using some function (I noticed it on noise reduction too) causes a tremendous slowdown.
I'm using a i72630QM with 8gb ram. It's fast enough to handle that stuff, at least it was on LR3.
Edit: just to clarify what I mean with "slowdown": the program is barely reacting anymore, takes several seconds to come back from minimize, pretty much feels like crashing nonstop.
I can say 99,99% certainty that its not hardware related.