Catalina Fan Solution
It’s not a 100% solution yet, but it’s the best so far: disconnecting my mac from the external monitor. More specifically, disconnecting the monitor from the Thunderbolt 2 port, and using the HDMI port instead.
For a while there, I started to see the kernal_task CPU usage hit something stupid like 600%+, which I’m not sure how that even works. Everything got slow and laggy, to the point where I was contemplating a fresh install from a Time Machine backup (still not out of the question).
So far, the laptop is functional again. And the fans are on, but less loud/fast. And there definitely seems to be less going on, looking at Activity Monitor.
I’m really regretting this upgrade. I figured this would sort itself out after a few days, but at least I have a working computer. My work laptop is on the verge of upgrading to Catalina, and I worry that I’ll be running into the same thing all over again.