@nviiw Do you have Eco Lifestyle installed, and if so, does it help to disable the effects of the eco footprint? You can keep testing in a new save if you want. And please test in one of the base game worlds.
It's understandable that fps would increase when you're viewing a basement because there's comparatively little background to render, and simpler lighting effects. But 40 fps is still pretty low for that situation.
I'd like to see the results of some hardware monitoring too, in case something else is going on. Download hwinfo from here:
https://www.hwinfo.com/download/
You don't need to install anything; just choose the portable version, unzip it, and launch it from Downloads or wherever you want. (If you'd prefer to install, make sure to click the button for hwinfo itself, not for lansweeper.) Restart your computer, launch hwinfo, choose "sensors only," and click the button that's a sheet of paper with a + to start logging.
Wait five minutes, then launch Sims 4 and play for at least 20 minutes; you can click the same button to end logging when you're done. Upload the log to the filesharing site of your choice (Google Drive, Dropbox, etc.) and link it here. Please leave the .csv in log format or create a .zip file, and don't open and save it or my log reader may not be able to make sense of the data.