Sounds like you might have anti-virus software scanning the email before it is sent.

If this is the case then, try switching off the scanning to see if this improves performance.

It is also possible that your Sent folder has become very large. Archive old sent emails, delete those you do not want, then right click on Sent folder and select 'Compact'.

It's happening on 4 different computers in our office and nothing has recently changed as far as settings on antivirus software. I have compacted my sent files and it's still very slow. I have looked everywhere on my anti-virus and can't see an easy "turn-off" setting. Our server says it's not them and wondered if Thunderbird had a recent setting change.

Test the issue by restarting your computer in safe mode with networking.

Change is not always originated by the user or the software manager. Anti virus programs phone home daily for updates these days. As does Thunderbird. Thunderbird tells you it is updating, anti virus programs assume you know.

cwright47 what were your results in safe mode?