If eg:Windows OS is performing an update, it may cause other programs to slow up.

Usually, when Thunderbird is not responding, another program is scanning or updating or you have Thunderbird running a load of preuse tasks.

Have you installed an Anti-Virus product or are you using the default one in Windows 10?

The default in Windows 10: Quote: Always-on protection scans emails as they arrive and as they are manipulated, just like normal files in the operating system. This provides the strongest form of protection and is the recommended setting for scanning emails.

There may be a time when you want to temporarily stop running real-time protection. However, real-time protection will turn on automatically after a short while to resume protecting your device. While real-time protection is off, files you open or download won’t be scanned for threats. Open Windows Defender Security Center, then select Virus & Threat Protection > Threat Settings. Turn off Real-time protection.

Helpful links:

• https://support.microsoft.com/en-gb/help/4013263/windows-10-protect-my-device-with-windows-defender-antivirus
• https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-antivirus/configure-advanced-scan-types-windows-defender-antivirus
• https://docs.microsoft.com/en-us/windows/security/threat-protection/windows-defender-antivirus/configure-real-time-protection-windows-defender-antivirus

Defender or AV product may be scanning Thunderbird program on start up and/or scanning Profile folders. I note you mention certain profile folders (emails are stored in the 'Roaming' > 'Thunderbird' folders) - are not included in AV scans but this is not the same as stopping real time scanning of emails. Defender or AV product may be scanning incoming emails or when moving emails and is usually the cause of non responsive messages.

First of all, thank you, Toad Hall, for the quick response.

Have now un-ticked Options > Security > Anti-virus : allow ... to quarantine ... messages ;

Windows won't be updating that many times a day, and there's usually a notification once done. Other updates all present a window that they'd like to update, then await my OK.

Pre-use tasks is not a known concept to me, assume that once TB started and loaded and HDD light stops the pre-use tasks will at some point have finished, and won't recur until I have closed and re-opened the program?

Here, the two-minute pause occurs intermittently, if I check mail again five minutes later , or half an hour later, the program may or may not stop responding.

Turned off AV real time, and collected pop mail, and still get Not Responding. The other two links in your mail are to Enterprise Security Managers who can set Group Policies in the Pro version, which I don't have - or did I miss a trick there, please?

If it were anti-virus I'd expect the delay to occur with some foolish consistency like 'always when collecting mail from each of the providers', but in this case there's rhyme nor reason, as downloading may go smoothly one time, cause multiple instances of delay the next?

And why do I not see greater activity in Process Explorer or Resource Monitor if it AV scanning were the culprit?

Yours still puzzled,

Here is a list of possible sources of performance problems, in more or less order of likelihood. One user recently found that corrupted global search indexes (no. 9) was the source, after eliminating AV, Windows integrated search etc.