Upload & Sell: Off
I installed Lightroom 4 (trial), and converted my catalog (18000+ RAWs 10MP). I regenerated standard previews. I enabled only library, develop and print modules.
After some moderate use (just occasional 1-2 gradients on image, some brush adjustments), I noticed significant slowdowns (non-responsive sliders, delayed refresh). On most pictures I adjust exposure, brightness, contrast, clarity, saturation and vibrance.
I checked memory usage - 1GB, max. 1.5GB. LR 3.6 was never a speed champ on my machine, but it was much better. Stopping and restarting the program didn't help, restarting the PC helped a little (apparently).
I like 2012 Process Version and workflow, but Adobe needs significant performance improvements.
My configuration: Win7 64bit, 6GB RAM, Intel Core2Duo E8400.