does the same problem occur if you create an exception in your anti virus program for Thunderbird's profile folder?

I don't understand what that is. I have Mcafee Total Protection including anti-spam but I disabled the anti-spam feature because it stopped working.

See https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

I got as far as this:

   In Windows, right+click on the McAfee icon in Windows notification area (aka system tray), or start McAfee from start menu
   "On-Access Scan properties ..."
   "All processes", "Exclusions"
   "Exclusions ..."
   Add the "Thunderbird" and "Thunderbird Profile" folders and subfolders. Folder names must finish with a backslash. NOTE, if you have customized profile or account locations for mail, then you must exclude your custom locations.

Where do I find "On-access scan properties"?