You may experience abnormally long processing times when exporting a VIM from Revit. Large VIM files (7GB+) will only take around 1+/- hour to export. If you are experiencing a longer time then you'll need to take these steps to fix it.
Close Autodesk Revit and any associated Autodesk applications that are not running.
Navigate to your %Temp% folder
C:\Users\YourComputerName\AppData\Local\Temp
Replace YourComputerName with the name of your PC
Select All folders and delete everything in this directory.
You might have to skip some files that are being used by other applications.
Reboot your PC.
Try to export a VIM file again.