For whatever its worth I have PM 4.6.6 running on Win7 64-bit, 12GB Ram, i7-930 CPU and it's rocket fast.
I just loaded 20k + images in a single contact sheet - load time 10 seconds.  Upon loading, I can scroll through the thumbs as fast as I can tap the down arrow key, if I go super fast I can get a little ahead of the program but the thumbs appear in less than 1 sec.  If I double click to produce a preview, I can then zoom through full size previews as fast as I can tap the arrow keys.  More ordinary size contact sheets, say 4-5k images or less, load and operate virtually instantaneously.
PM could not load or run fast with big contact sheets on my prior computer running 32-bit XP Pro with 4GB RAM but with small contact sheets, say 1k images or less, it was just as fast.  So I don't think Win7 has anything to do with PM's speed, its rocket fast on XP and Win7 (never used Vista).  The huge increase in RAM available in 64-bit Win7 allows me to work with much bigger contact sheets and still run rocket fast.
Bill Wood