What is your anti-virus software?

Try to start Windows in safe mode with networking enabled.

• Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode
• Win8 https://support.microsoft.com/en-us/help/17076/windows-8-startup-settings-safe-mode
• Win7 https://support.microsoft.com/en-us/help/17419/windows-7-advanced-startup-options-safe-mode#start-computer-safe-mode=windows-7

Does the problem go away?

Christ1,

Thank you for responding. I use the standard Microsoft MsMpEng.exe.

I will try the Safe Mode but I will not necessarily get an instant answer. The problem is frequent, but not present all of the time.

I see the suggestion to run in Safe mode frequently, but do not know what that does or what it proves, and hopefully it is not the "solution" per se.

Thanks again.

I use the standard Microsoft MsMpEng.exe.

What is this? Windows Defender?

I see the suggestion to run in Safe mode frequently, but do not know what that does or what it proves

Safe mode is not a solution, it's a troubleshooting tool.
It's the only reliable way to disable anti-virus virus. If it works in Windows safe mode anti-virus software is most likely the culprit.

Christ1,

Thank you for reply. Microsoft (MS) has "Security Essentials" and "Windows Defender". I have the former. MS says that they do the same thing. https://support.microsoft.com/en-us/help/4466972/windows-10-antivirus-and-antimalware-software-faq

I have always used the MS antivirus only.

I will try the Safe Mode and report back.

Thank you.

Dear Christ1,

Want to thank you again for attempting to assist with this situation. After a few days of poking around in the BIOS, I do not find Safe Mode. The microsoft article that you referenced suggests a function key press that is not one that works on my HP desktop machine.

Anyway, guess I'll just chalk it up to another app-OS-incompatibility .......

Do you have any suggestions for other good mail clients?

I have experimented in the past with Claws mail, but found it difficult-to-impossible to get it to work with SSL-TLS access to POP3 accounts.

All the best and thank you again.

Safemode is a function of Windows, NOT BIOS. Shutdown your computer, then power it up and immediately start pressing the F8 function key repeatedly at a rate of twice per second until the the Advanced boot menu appears. The idea is to press the key and have it registered by the system BEFORE Windows starts loading normally.

Thanks Stans,

I have already done what you suggest, numerous times.

I simply get the usual Windows boot screen.

F10 gets me into the BIOS, so I am confident that my keyboard is indeed sending the key codes through.

Not sure why F8 does not seem to work.

Thanks again for your post !

Alternatively, use msconfig to set Windows to boot in safe mode next time it starts. See details here https://www.lifewire.com/how-to-start-windows-in-safe-mode-using-system-configuration-2626115 or https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjdi761nPXpAhWRx4UKHW0uA74QwqsBMAt6BAgKEAQ&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3Dt8sJ9eQF6AY&usg=AOvVaw3luR8inLd5wldL-bqhGbt8

All,

Thank you for your assistance. After several days with Safe Mode, with various CPU and memory loading, I can report the following:

-- Response in Thunderbird to various mouse actions is consistently faster -- On rare occasion, jumping (mouse selection) from a folder in an account to a folder in the Local Folders hierarchy, will take 2-3 seconds, but never more than that. -- No occurrences of "Thunderbird Not responding" in the top menu bar.

Thank you,

No occurrences of "Thunderbird Not responding"

Windows safe mode disables anti-virus software, which is most likely the culprit. These are some generic suggestions to avoid problems with anti-virus software.

Create an exception in your anti-virus software for the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it.
http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Don't let your anti-virus software scan incoming and outgoing messages.

Don't let your anti-virus software scan attachments.

Don't let your anti-virus software intercept your secure connection to the server.

Remove any add-ons your anti-virus software may have installed in Thunderbird.

Keep it working.
http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

And last but not least, backup your Thunderbird profile on a regular basis.
https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile

Thank you Christ1 and Stans for all of your assistance !

All the best to you.