LR3 is indeed an excellent app but, as a current LR3 user who has run into the odd slowness that came only after the official release version which has been somewhat addressed in LR3.2 but not fully, I would never tell anyone to jump into LR3.x feet first without a 30-day trial, especially using the adjustment brush as well as the spot tools. This is where my particular system is bogging down if I am not observant. it is a memory issue because closing down the program then opening it back up lets me work for another 10-mins or so on the same image before I start to see issues again.
And it has been shown there is not a hardware barrier but some other confluence of factors as yet unidentified by anyone because identical hardware can have the same issues. I even found a few folks who had side by side twin systems so to quiet the naysayers they swapped RAM as one system worked FINE but the other had problems...and many folks were adamant it was the "poor" quality of the RAM, that got blown out of the water, then they swapped CPU's leaving the RAM swapped and nope that was not it...ditto for the graphics card....one by one they swapped hardware save for the HDD...but a clean Win7 x64 Ultimate install did nothing to improve the issue. Plus the issue crosses platforms. Soooo, for LR3.x I say running the trial very hard for 30-days is extremely important in case that person is one of the "lucky" ones...it's frustrating for me because I LOVE the new LR rendering and editing, it is slicker than snot...but, I have to be cautious with a few of the very tools I love the most.
BTW, those same tools work fine in ACR 6.x but because LR3 does things on an, apparently, higher level there are caveats...