Step by step at https://wiki.mozilla.org/Thunderbird:Testing:Memory_Usage_Problems

Thanks for your response, Wayne. Re the diag suggestions: My T-bird (Windows 10 client) is on the latest version (128.2.3esr (32-bit)). The slowdown in T-bird functioning started after recent updates. I checked add-ons. Only two. One I added awhile back (Compact Headers) and another that I don't think I added (CompactHeader). Not sure how or when that latter one was added. Perhaps in a recent update? I disabled CompactHeader and restarted T-bird. So far, it appears to be running as expected now - it's not taking forever to load a message, switch messages, delete a message, or get messages. Sorry for the drama. But that slowdown was so abrupt and recent that it really threw me off after years of satisfying use of T-bird. If the functionality continues successfully, I will donate again. I would like to know how CompactHeader was added. It wasn't me, obviously, since I already had Compact Headers. Thanks again for your assistance, John

Thanks for that interesting information.

https://addons.thunderbird.net/en-US/thunderbird/addon/compactheader/ is the predecessor to https://addons.thunderbird.net/en-US/thunderbird/addon/compact-headers/. These can only have been installed somehow by you.

compactheader has been "not compatible" for a long time, so I doubt you'd even be able to install it on a current version. Interesting that it's presence, even disabled, could have a performance effect.

Before doing anything with compactheader, had you tried Thunderbird troubleshoot mode from the Help menu? And did it solve the performance issue?

Disabling CompactHeaders sped up T-bird. I will try the other diag measures later to see if it helps the less significant residual memory issues. I rarely do anything with add-ons, mostly to avoid problems. The last time was at least a year ago. The slowdown only occurred after the last two recent T-bird updates. I always do the updates in a timely manner. The program worked efficiently until then. I realize that you're the expert. But I haven't done anything with add-ons in a long time. Thanks again, John

Wayne, you were right - that did not solve the problem. T-bird moved a little quicker for a short time, then it went back to that incredibly slow pace. I will have to work through all those troubleshooting tasks. There are no network, memory, disk, or other such issues, based on the Windows Task Manager. Every other program works fine. This issue only started after recent T-bird updates. I have to assume that T-bird programmers adjusted how the program utilizes memory. It's a bummer after 22 years of satisfied use. In any case, this issue is not solved as yet.