I'm not asking a question now; I already found the answer. I'm contributing an answer in case others run into the same problem I did. The archive function in Thunderbird was running painfully slowly. I remembered it as being fast, moving from local folders (from POP) to local archive folders quickly, but lately it had been slow, like seconds per message. And I had thousands of messages to deal with. This is on Windows 10 Pro.

The fix turns out to be in Settings -> Windows Security -> Virus and Threat Protections. It has an option "Cloud-delivered protection", which "Works best with automatic sample submission turned on". Now frankly that sounds like a major-league security hole to me, sending old private messages to Microsoft! Turning them both off fixed the problem. Now archive is entirely local and isn't held up by Windows mucking into the system and trying to make copies of private mail and send them to the mothership before moving them. I don't know how Windows "Threat protection" broke into the internal operation of T'bird but that was an easy fix once I found it, and the archive function is very fast, even moving thousands of messages at a time.