One thing you may want to look at is to get the application Onyx, from here and run it to clean up your system. You didn't mention the size of the HD, or how much space is left on it, but on a relatively new machine unless you loaded it up that shouldn't be a problem. One other potential area of slowdowns could be that whatever you are doing with the 20 page document is causing Word to write out the document to the HD to preserve the history, and that your HD is getting full, or the scratch space Word may be using is getting fragmented.
If it's only showing up for the Word application, I would suspect some component of MS Office is not performing properly and not getting back to the OS when it is done, if it is, in fact, completing the task. The spinning beachball is the OS saying it's busy, or waiting for something to complete. About all I can suggest is to uninstall and reinstall Office. Nothing there out of normal, plenty of memory.
