When did you last run Disk Cleanup and the Defragmenter? Operations like you describe need large amounts of contiguous free hard drive space.
How fast is your processor, how much RAM do you have and how much free, clean, defragmented space is on your hard drive(s)?
Also, where is your project file (.prel) saved? Is it saved inside its own directory folder? Try doing a Save As and saving your project under a new name, in a new folder and see if that makes any difference. (After you've cleaned up your drive and defragged, of course.)
In addition to Steve's very important questions, I have another:
What else is running on your computer (often launched "behind the scenes," at bootup), while you are editing? Most computers have a lot of unnecessary programs, and processes, that launch at start, and can really eat up computer resources. You can check to see what gets loaded with MSCONFIG, and then looking at the Startup and Services Tabs.
Good luck, and please let us know a bit more.