Hmm! your post has puzzled me. I have the base i5 iMac with an upgrade to 8 gig of RAM and I too run Parallels 6 with Windows 7 VM. I have dedicated 2 gig of RAM to the VM along with 2 CPU's. I don't run in coherence mode as I like to keep OS X and Windows as separate entities. Set up like this I find no lag whatever I'm doing OS X or Windows. I would heartily recommend Parallels to anyone wanting to use Windows as a VM on the Mac. Perhaps this is where a Desktop scores over a Laptop - just that bit of extra grunt where you need it?
What else do you have open at the same time and what are you doing in windows at the time.