Tahoe seems to have slowed down the OS a bit but I don't think it is as extreme as you're seeing here. I don't have any intel chips to test with myself but personally I'd try running some benchmarks both CPU and GPU to see if your scores match up with other people running your hardware. After that maybe check to see if you're throwing errors in your console. Maybe it'll lead to a hint.

It seems like a bad bug somewhere rather than a purposeful event

I'm confident it isn't purposeful. But no hardware errors can be found.

Virgin install Tahoe: grinds from 74-C up to 100-C within a minute and then throttles. Virgin install Sonoma: runs fine. No temperature change of note.

This isn't indicative of a hardware problem.