It churns on my two systems as well, but I believe Frogboy mentioned that the beta has all debug settings maxed so that they can gather data. These settings will be turned off, or severely lowered, in the final release and the impact should not be as great.
I noticed that on my laptop (core-2-duo T7200 - 2GB Ram, NV 7900GS 256 - 320gb 7200rpm) the game suddently slowed to a halt today similar to your computer. So, I am wondering if the new xml settings we are downloading are somehow tweaking something that has really bad impacts on the code. Windows 7 - 32bit
Desktop runs fine though and smooth as a bell... ( two physical Xeon 5160 processors ( 2 x c2d 3.0ghz equiv) - 8gb ram - ATI 4890 1gb - 72gb 15k raptor) Windows Vista Ult 64bit.
By the way.. the desktop takes anywhere from 30-60% cpu across ALL CPUs and 700-1000 MB of RAM. When I look it is floating over 50% more often then not.