Few things to check...

Slow loading if CADmep can be due to validating service buttons. You should see a progress bar in the lower right. Hitting ESC cancels this process. Or use the Edit Configuration utility to disable the button validation.

File size can be drastically increased if the "Save with Graphics" options is enabled. This should be OFF unless there's a reason to have it enabled. This is in your Fabrication database. Setting can be found here...

Also check the ISAVEPERCENT system variable in AutoCAD. AutoCAD doesn't always save the entire DWG when you click save. It just appends to the end. ISAVEPERCENT is the % of "wasted" space that can be in a DWG before a full write is performed when saving. e.g. If you delete an object in AutoCAD, saving your drawing won't always make it smaller, it just records the deletion and appends the info to the file to increase save times on large file.  I'd suggest any setting over 50 (default) is problematic. 

Check your CUI Editor for any unresolved menus or partial menu's. This can slow launching and loading files w/CADmep.