Thank you very much for the suggestion. Disabling FlatUI did not solve the issue, but disabling ColorUI did. I will post a comment on that mod. Having FlatUI enabled while ColorUI was disabled, also allowed for a smooth framerate. I will simply keep ColorUI disabled going forward. Thanks again!

What did you do?
I open my inventory menu.

What happened?
The framerate craters from a solid 60 down to 17 or so, even on an entirely fresh, new run. It remains this bad while the inventory menu is open. It does not matter which tab I'm on. When I close the inventory again, the framerate recovers ...