What is your anti-virus software?

Microsoft Defender Antivirus

Try to start Windows 11 in safe mode with networking enabled.

Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

I do not get the slow deletions and hangup in Safe mode. But Safe Mode disables a number of things, not just anti-virus software. So how do I pinpoint the actual responsible software? Also, I am running Windows 10, not Windows 11.

Most likely anti-virus software is the culprit here.

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

Oddly enough, just going into Safe Mode & restarting seemed to solve the problem. I also backed up my profile to an external HD. And I'm also in the process of creating an exception in my antivirus software for the Thunderbird profile folder. I have no idea what actually fixed this issue, but I'm so happy that deletions are now fast as usual. Thank you for all your suggestions!