What is your anti-virus software?

Bitdefender and I have made Thunderbird.exe and the Thunderbird folder exclusions, but it has made no apparent difference.

What is weird is that if I kill the process and then restart, it behaves almost normally for a few clicks and then typically goes into forever mode with "Loading message..." as the most common state, then nothing else works until it finally comes back (if it ever does).

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 have made Thunderbird.exe and the Thunderbird folder exclusions, but it has made no apparent difference.

There is no point creating an exception for the program folder. You should create an exception for the Thunderbird profile though. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Actually it was the Profile folder I meant. I have my actual email files in a different folder and have excluded that too. No difference. I have turned off all AV checking - no difference. I get about 3 minutes after a restart when it works fine - then it goes silly. Just for background, I have used Thunderbird for over 10 years with zero problems and good performance. Something has changed dramatically. Maybe I should just try going back to an older version. This one seems completely screwed.