Since this seems to be isolated to one project. I would focus on the XREFs.
First off, the long lag could be because AutoCAD is searching for stuff it cannot find. Check the XREFs for any XREFs that you do not have. Things like images are often not included. If you have any nested XREFs, I would try removing them and reference them at the first level.
If the XREFs are from an outside source and that source could be using a vertical product that you are not using, you need to make sure they have properly converted. Get Drawing TrueView free from Autodesk and convert all the XREFs to your version of AutoCAD.
If, after doing the above, you are still having issues, giving the XREFs a thorough cleaning and examining them for anything that might be unusual. Start with looking at the command line history after opening the files. AutoCAD might give you messages if it is recognizing any issues. Hitting F2 should bring up a separate window that you can scroll through the opening messages. Next examine the contents of the file for anything unusual. I've had a single attribute that was not associated with a block bring referencing files to a slow crawl. Large hatch patterns or large amounts of hatches can also be resource hogs. If you are not familiar with how to clean up a drawing, there are many resources for additional steps to take or LISP routines to do a brute force cleaning if you are not interested in learning what is causing the problem.
If after that, you are still having problems, you might want to take a look at simplifying the drawing by deleting unnecessary content, flattening 3D content, etc. Exploding everything or anything is usually not a good idea. Blocks actually help with performance.
Rob
Drafting is a breeze and Revit doesn't always work the way you think it should.