Folders in Thunderbird are actually files on your file system. When you have lots of messages in a folder, the file gets bigger. Probably each time you send a message your virus scanner kicks in and also verifies the big file.

You can move older emails from your sent folder to Gmail's All Mail folder, that probably improves performance.

But you can also configure Thunderbird not to save outgoing messages to the Sent folder. Gmail will do that itself, so when Thunderbird does it too, it will actually happen twice. Go to Tools -> Account Settings -> <Gmail account> -> Copies & Folders and uncheck Place a copy in...

Thanks very much for your suggested fix. I have dozens of local folders in Thunderbird that help me track client projects, etc. But those same sub-folders do not appear in GMail. So it would not help me to only have messages copied to GMail's "Sent Mail" folder. Under "Program Files" on my computer, I can see that many of the sub-folders in Thunderbird are quite large, which could account for the delay in copying messages. But if I still need immediate access to these sub-folders, I can't really delete the contents to reduce the file size. Is there some way to synch Thunderbird and GMail so that all sub-folders are re-created in GMail?

Onno Ekker said

Folders in Thunderbird are actually files on your file system. When you have lots of messages in a folder, the file gets bigger. Probably each time you send a message your virus scanner kicks in and also verifies the big file. You can move older emails from your sent folder to Gmail's All Mail folder, that probably improves performance. But you can also configure Thunderbird not to save outgoing messages to the Sent folder. Gmail will do that itself, so when Thunderbird does it too, it will actually happen twice. Go to Tools -> Account Settings -> <Gmail account> -> Copies & Folders and uncheck Place a copy in...

We have a very recent Windows 8.1 desktop with light personal use only. lots of storage.

The slowness of sent messages being saved never appeared in a 10 year old XP sytem we used formerly.

So no we do not have that many folders!