Unequivocally: it happened when I upgraded from macOS 11 to 12. I had saved a full bootable image backup of 11.1 on an external rotating USB 2.0 disk. Booting takes about half an hour and applications take minutes to open.
Running Xojo 2020 Release 1.2 under macOS 11.1, I was able to open an 11/19/2020 version of my project in the IDE, and (once loaded), the offending window display and loaded in about 3 seconds. Normal.
Loading the same version of the same project under the same version of Xojo, under macOS 12.2, displays the same old 10-or-so-minute load time. 10 minutes under macOS 12.2.1, three seconds under macOS 11.1.
I can run the app under 11.1 and it, too, behaves “normally,” with the offending window opening in a reasonable amount of time.
I don’t know where to go with this, but the problem is definitely associated with Monterey.