FWIW, I believe it is some quirk with 4.6.5.
I just uninstalled .5, reinstalled .4, and am having no problems. Sheets load quickly, like they used to, no hangs or other problems. With .5, a folder of a few hundred NEF's & JPG's was taking 2-3 minutes to load, and one of a couple thousand took, well, I don't know because I always ran out of patience after about 15 minutes. Previews also render quickly now, whereas they were taking a while with .5.
My machine is a Dell with Windows 7 x64, an i7 Quad at 2.67 GHz and 6GB of RAM. I've got - per previous suggestions - my PM settings in cache at 1024 for disk cache and 768 for memory. My images are stored on a USB Drobo with 4 TB of RAID-1 storage.
Thanks,
Jake Norton
MountainWorld Photography
www.mountainworldphoto.com