What is your anti-virus software?

christ1: I actually use two: NORTON 360 and MALWAREBYTES. Although I would have to say I didnt seem to have the "Thunderbird not responding" popup in the previous latest version of THUNDERBIRD; it only appeared after the latest upgrade. It seems not to be consistent; some days or during the day it, THUNDERBIRD, will work quickly when selecting emails for deleting..then the next day or later in the day I will select emails and try to delete them then it seems to hang and I get the popup. I see on the sidebar here SEE ALSO: "Thunderbird slow, unusable. CAUSE: Windows/Microsoft Defender" I will take a look at that link and see what it says. Thanks.

Using three anti-virus/malware scanner simultaneously is a bad idea. I'd settle for Microsoft Defender and get rid of Norton and Malwarebytes.

These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile