Hi Donald,
as far as I understand Parallels is creating a file at runtime that contains pretty much the memory of the VM, i.e. its current state. It updates that file every once in a while to reflect the current memory's state. The main purpose of that file is that when you put the VM (or the Mac) to sleep, it can relatively quickly flush its memory to disk. However, the more memory you assign to the VM, the larger that file is, and the more data the Mac will have to write to disk before it can go to sleep.
I've found that Parallels - and Fusion, too, for that matter - don't interact that terribly well with the Mac's virtual memory. That means that any disk operation that has to do with the VM - accessing the VM's virtual memory, or flushing memory to disk in preparation for sleep - may clash with the Mac trying to swap. From what I read here this seems to be relatively independent of the physical memory installed in a Mac; when the VM exceeds a certain size, the Mac starts swapping and the VM become slow. (People here who have 16 Gig in their machines, only 4 or so of which are assigned to the VM, have been complainging about slowdowns.)
What happens if you try to set your VM to sleep manually before you close your Mac's lid? I.e. use the "sleep" feature from the VM menu. Is putting the VM to sleep manually also slow? And what about the Mac's speed-to-sleep after you've put the VM to sleep?
I've reduced my Windows 7 VM size to 768 MB which is sufficient (just about) for day-to-day work, and sleeping isn't an issue here. When I had the VM at > 1 GB it certainly was an issue.
Good luck!