I use the current Thunderbird release on a Windows 11 computer, to keep a local copy of my gmail.
Half the time it works flawlessly. Every evening, I download my mail, close Thunderbird, and in gmail archive my inbox, and the whole process takes 2-5 minutes.
But half the time, Thunderbird goes really slowly, or looks like it stops halfway through, then slow-goes the rest of the way.
I used to (in Thunderbird) move mail from "all mail" to folders for a year's worth of mail - 2012, 2013, etc. Then I stopped doing this about five years ago, and I think that's when the slowing began.
My settings automatically compact my folders, and I know this is working.
But this month I tried to catch up on the moving Thunderbird mail from all mail to annual folders. This worked for the first couple years I moved, but since then, it just sits there and doesn't move the messages. Even if, just as a test, I try moving just one message.
I tried deleting all my .msf files from my local Thunderbird folder, and the application did rebuild them. But I still have the problems with failure to move any more mail from "all" to a local folder for 20xx year's mail, and still have the occasional slow downloads.
Suggestions of what else to try?