Sounds like the Anti-Virus is scanning opened files which occur upon download and it's likely the AV is also scanning anything you download and upload. When an Anti-Virus intervenes in the process there is delay and Thunderbird is temporarilly unable to access it's own files.

How to test properly: You need to exit all programs and then restart the computer in 'Safe Mode with Networking' - note if you normally use wifi to connect to internet then you will need to connect your computer to the router/hub using an ethernet cable before you restart in 'Safe Mode with Networking' mode. Then start Thunderbird as normal.

Alternatively - set up Anti-Virus exception to not scan any file in the thunderbird profile folder. If using Windows OS and profile is in default location then you need to set to not scan this 'Thunderbird' folder or any file or folder within it. C://Users /username/Appdata/Roaming/Thunderbird

Please note it will also help if you regularly compact your folders especially Inbox, Junk, Drafts as they get the most action when it comes to moving or deleting emails.

Excluding the Thunderbird folder from Microsoft Defender Anti-Virus scans seems to have done the trick. Thank you, Toad-Hall!

By excluding that folder from scans, have I compromised my security in any significant way?

Excluding the Thunderbird folder from Microsoft Defender Anti-Virus scans seems to have done the trick. Thank you, Toad-Hall! By excluding that folder from scans, have I compromised my security in any significant way?

Not really. Emails in themselves are harmless. They may only get 'dangerous' if the recipient - aka you - does something foolish.

When you download emails nothing happens even when you open the email - it depends upon what settings you allow and what you do next which matters. But there are a few basic rules to follow for any email and if you follow them you'll be ok.

Never assume it is from the person it might say it is from. Some nefarious people use other peoples email address in the FROM but it's not really from that person. Sometimes the Display Name looks legit but the email address is clearly wrong. Usually, you can tell if the email is from a personal friend by the way they type the email. Sometimes the 'Reply-to' says it's to a completely unknown email address.

Thunderbird by default does not allow any remote content. Images may have remote content, so if you see black outline of a box with a broken file icon then it's an image containing remote content. Sometimes that remote content is just linking image to a website and if you allow it to display, it's possible the sender will know you have opened their email to you and they know what info/image content interested you. Nothing more sinister then using a browser and searching the internet or viewing legit websites. Sometimes the icon is so small you would not know it exists and the remote content may be a tracker. Othertimes clicking on an image or text link may do sometime you do not want such as directing you to a website which is harmful and could attempt to download something you do not want.

Useful additional info: https://kb.mozillazine.org/Privacy_basics_-_Thunderbird https://support.mozilla.org/en-US/kb/remote-content-in-messages

So the Advise is -

1. Never assume it is from the person it might say it is from.
2. Do not change settings to allow remote content.
3. Do not auto switch on 'Display Attachments inline' until you are certain who sent the email and it contains image attachments you want to view within the email - not using using another program. Disable the option after use.
4. Never click on any link - image or text - until you know what you are clicking on - hover over link and the real link information is shown in the bottom Status bar.
5. Never open any attachment until you know who sent the email. Do not be curious and try to open something just to find out what it is. There are plenty of bogus emails from people saying eg: your Invoice is attached, when you know you never ordered anything. Malicious attachments are disguised as important documents, invoices, advertisements, or other seemingly legitimate files to entice users to open them because people are curious.
6. If you get suspcious emails - delete them and then compact the folder to remove all traces of the deleted email.

Problem is back. Seems to be particularly bad whenever Thunderbird is retrieving messages. I didn't have this problem in earlier versions of Thunderbird.

Does change sort help as described at https://bugzilla.mozilla.org/show_bug.cgi?id=1908761#c5 ?