Those are, for lack of a better word, 'built-in'.

Thanks, David. I tried running TB in Troubleshoot Mode on, but that made no difference to the slowness of downloading messages -- so Add-ons and Extensions aren't the problem there.

Downloading a message in the inbox can take a full minute now, and 'sending' a message can take the same while it's put into the sent folder, the little blue whizzy turning all the while.

If it's not Add-ons causing the slowness, do you have any idea how to resolve the problem?

Thanks again, Gail

All I can say is that it sounds like a network problem. Either a slow server or a heavily-loaded one.

'Slow' performance can be due to a number of things, but the first one to check is the effect of antivirus/security apps. What is yours? The TB profile folder should always be excluded from AV scanning (look in AV settings). As a test, run Windows in safe mode to bypass startup apps, like AV, that might degrade TB performance.

https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems