It is not Thunderbird who needs to work with Avast. It is the other way around as is the case with all AV software. Unusual as Avast is generally ok.

Avast is taking over control and scanning, so thunderbird has to wait.

Do you have another AV product on your computer, even a default one which came auto installed that could be running as well?

Some useful info: https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#AVAST

Perhaps removing Avast will fix the problem:

https://support.mozilla.org/en-US/questions/1176496#answer-1010336

Thanks guys. No, I don't have any other AV programs. I've tried removing and re-installing Avast, and also Tbird, with little success. If I remove Avast, I have to find another AV program which doesn't cause the same problems. What surprises me is that it worked perfectly until a couple of weeks ago, then fell apart. It may have been an auto update of one or the other.

If you keep Avast, or any other AV program, you should set it to not scan the TB profile folder; the real-time background monitor is sufficient to guard against infected attachments. To test if an AV program is causing performance issues, run in Windows safe mode, as explained in the wiki article cited above.

If the AV program is not the cause, this article details most of the other possible sources of performance issues.

I've set Avast to ignore the profile folder, but it's still slow. I'll look at the other article you cite to see if there's anything there which will help. Thank you for your help so far.

Tried Avast free once or twice, really "expensive" GUI and too intrusive scan services (they want to cover all areas and inspect all protocols so they proxy everything).

If it is the software of choice for you then go try what they've said. Put an Exclusion to the TB profile folder, maybe to the TB install folder too as it maybe monitoring the EXE behaviour. Make sure all options about SMTP, POP, IMAP or EMAIL references are DISABLED so it does not scan or tries to interfere with TB when connecting to the server.

If not your favorite, I'm using Avira Free which is becoming bad with time and Bitdefender as a paid solution which after you disable some intrusive features it is a really good product. None of them gave me any trouble over the years.

PS: Forgot the most important thing. If you uninstall AVAST, TB works fine?

I think there may be problems with something else. I'm going to try re-imaging my operating system, and see if that fixes it. I just need to prepare for it so I don't lose anything (I have my data on separate RAID discs, so I can restore my operating system). I suspect I'm getting problems because I'm running WIN 7 on a new hardware system I built, but it's not designed for WIN 7, so I can't get updates. Although I'm not sure what could be causing this problem because of that.