Do you use an anti-virus product to scan outgoing mail?

If yes, switch off the scanning of outgoing mail.

Report back on whether this improved the situation.

I did as you suggested, but it still took a long time for a simple one line reply (without quoting the received message) to go through. It didn't time out this time, but I'll keep monitoring it.

So far, the only improvement is that I'm not seeing messages reported as not sent that have actually been sent. I get alerts from Norton about not scanning my outgoing email, but the 'send' is still very slow.

All different "modes" of my thunderbird randomly go very very slow. Deleting an email, small with no attachments, can take over 1 minute, then usually the next ones are fast, but not always. Same for sending. Same for copy to sent folder. I use gmail.

I have been having the same problem on my desktop for a few weeks. It may be coincidental but I upgraded to Win 10 about then. I don't have the slowness on my laptop which is still Win 7

No to win 10, it's been slow before that even came out. Thunderbird can be painfully slow whenever it decides to.

I am also having this problem. It began about a week ago. Never had it before. 99% of outgoing messages I get a block reading something to the effect copying message to sent folder. The little wheel just spins and spins. Then I eventually get a message saying there was an error in copying to sent folder. I check sent folder. No message at first, then after some minutes it appears, some times, maybe 75%. Some messages never appear in the sent folder. As this began recently, I suspect a bug in a recent Thunderbird update.

Things that can make Thunderbird appear to go slow:

Background processes on your computer such as : Windows updates; Adobe updates; Anti-virus updates; etc - I'm sure you get the idea.

Anti-Virus products: scanning Thunderbird files and folders Scanning of incoming and outgoing mail.

Not compacting the Inbox folder often enough, so leaving files with loads of marked as deleted emails. eg: I compact the Inbox and Junk at the end of each day.

Hardware acceleration can cause issues for some. Not having your graphics card drivers up to date IMAP folders synchronising with server - some servers are not so quick to update and therefore can seem like a delay in getting the results in thunderbird,but then imap folders are a remote view of folders on server.

More info at this link; http://kb.mozillazine.org/Performance_-_Thunderbird

If that is the case, why did this happen all of a sudden. My computer today is the same computer of 10 days ago. No additional programs, same virus scan, regularly cleaned of unnecessary files. It seems that others are having the same issue, and their issues started just recently as well.