What is your anti-virus software?

McAfee. Regardless of what anti-virus software is used, it worked just fine before the upgrade. When I select an email with large jpg attachment(s) the body of the email initially shows the image(s) at full size, extending beyond the email viewing window. After chugging for a while - a long while - the image(s) are rescaled to fit and I regain control.

As a troubleshooting step, try to start Windows 7 in safe mode with networking enabled.

Does the problem go away?

No, the problem does not go away in safe mode.

Can you confirm you did start Windows in safe mode?

Yes, I did.

So these are large images, I would check your system temp folder is not clogged up with a million old lost temp files. I have seen windows grind to an almost stand still creating new temp files (which each image will be) when the temp folder is cluttered.

Yes, these are large image files, and multiples different files are attached to each email. The TEMP folder is not clogged up with a million old lost temp files. And, I did not have this problem until Thunderbird was automatically updated to version 91.

And, I did not have this problem until Thunderbird was automatically updated to version 91.

Yet I have is installed and it is running faster and starting more quickly than it's predecessor. So I am looking for reasons why yours would no be operating the same as mine. It may just be you are using an ancient copy of windows. But windows 7 and 10 are not all that different.

Or it might be your anti virus can not cope with the task and is slowing things down. Given the long an troubled history Thunderbird has had with McAfee (dating back to a new heuristics engine in 2012) I really feel that is quite probably the place to start looking, despite your test in safe mode not indicating the problem is resolved.

The normal approach is to start Thunderbird in troubleshooting mode (hold shift while starting) if that does not help try the operating system safe mode and while in that mode try the Thunderbird troubleshooting mode again. Each test offers clues as to the possible causes and solutions. One thing I can assure you is that despite your issue being with Thunderbird having a close Thunderbird is the problem approach to finding a solution will make getting there exceedingly difficult. Mainly because most post update issues are caused by anti virus programs that are so tightly integrated in the operating system they mess up on the version change to Thunderbird, either not recognizing it at all and blocking it, or scanning it's files like as if it was a word processor. Or trying to inject their code into a new version and having it cripple the program (scam and spam tools top that part of the list)

I will start both Windows and Thunderbird in safe mode as you suggest at next opportunity.

Let me also observe that when the email is selected for viewing, Thunderbird automatically displays the image attachments. In the bar below the Thunderbird window the words "Loading Message..." appear along with a narrow blue bar indicating the loading process occurring. Perhaps that will help with diagnosis.

When Thunderbird "strains and strains", can you have a look at Windows' task manager to see what process is particularly busy? From memory on Windows 7 you'll probably want to tick the "show processes from all users" box.

That might help confirm if (as suspected above) it's the antivirus that is the issue.

Task Manager indicates that Thunderbird is chewing up cpu for extended period.