I was having the same issues, and came looking for an answer...and I may have figured out one solution on the way over
I have a 2010 Mid Summer I7 macbook pro, and had been running windows 7 in fusion with zero problems since last fall. I upgraded to lion when it came out, and like you...everything began to crawl...I hadn't thought much about until this afternoon because I only use windows for a few programs for school, and I was trying to use it today for the first time this fall.
Anyways, check to see if you have auto graphics switching checked in energy saver under your system preferences...as soon as I unchecked it things started to speed back up in about 10 seconds, and now it seems to be acting as it should!
It's not a perfect fix, but it may help speed things back up for you! (RAM is dirt cheap too now, and might be another solution)