when you click on a message, a temp file is created in the system temporary folder called nsmail what can happen if you have a sluggish anti virus is it takes an age to scan the file so Thunderbird grinds along while the anti virus does very little. I suggest you create an exception in your anti virus product for files names nsmail.tmp and nsmail.htm

Thanks for your reply. I searched my whole C drive, but neither of those 2 files was found. I'm on Windows 10. Any other suggestions?

cupojo said

Thanks for your reply. I searched my whole C drive, but neither of those 2 files was found. I'm on Windows 10. Any other suggestions?

They are created when the messages are being read asnd deleted as soon as they are done. Locating them will not help you. Creating an exception for them might.

if you do not know where windows hides your temp folder. Hold the windows key and press R Type %tmp% in the dialog and press enter

Windows fild manager will open in the temp folder which windows does not sear anyway. Windows search is really rubbish for locating anything but Documents and Pictures or perhaps some music and video. It simply does not search probably 90% of the locations on a hard disk because they contain "system" files or other stuff Microsoft thinks you are not smart enough to know about.

OK i think you are saying to add the whole directory? I've added C:\Users\JoeS\AppData\Local\Temp to the exclusion list. Using Windows defender security center for anti virus. I'll let you know how it goes thanks.

There are many possible reasons for slow performance, but I don't think Windows Defender is at fault in this case. Consider some of the steps suggested in this post:

https://support.mozilla.org/en-US/questions/1234832

If you are going to add an exception to Defender, the logical choice is the TB profile folder, usually located at

C:\Users\username\AppData\Roaming\Thunderbird\Profiles\random textstring.default