How can I get parallels 6 to run well on my MBP
I had the same issue. Moving the Parallels virtual machine to an external hard drive completely changed it for me, from terrible to tolerable. 

I'm using an old laptop hard drive, 5,400rpm (so not even a fast one), and it works fine.

All you need to do is plug in the hard drive,drag the Parallels virtual machine to the hard drive, and double click to open (or go through the Open virtual machine dialogue in Parallels.