Suggest you move to the final 4.1. Amazing speed up of sliders response and rendering after adjustment brushes.
It's true that bashing a product based on betas and release candidates is not appropriate. However, even if LR4.1 does show some improvement, the inexplicable slowdowns forcing a restart still happen - to catch up with LR3 stability, I guess we'll have until LR4.3.
I saw a dramatic improvement. I don't have the fastest system, but I have tried to tune it to LR needs...i.e. fast processor, Win7-64, >8GB memory, SSD for OS, RAW cache, and LR Cat.
With 4.1RC2, I only had...significant...problems when doing lots of adj. brush work....which did cause, at times, the need to restart as processing time and memory usage increase...a lot.
LR 4.1 final has clear all this up. I know only see minor delays with LOTS of adj. brush work....no problems with NR on, second monitor active, nor, really, anything else. Have never needed a restart.
You seem to be aware of the current things to do with 4.1 to reduce processing issues. If you are having the problems you say, I suspect other issues in your system. What config and setup do you have?