Thank you very much for your response.
I do always play in a windowed mode, and I had already tried clearing cache, shortcut factory reset, clearing cache and repair, and I have zero mods. It helps to alleviate the lag / frame dropping for a while, but then it slowly creeps back.
It seems to be related to how bulky the save is though (like when my Sim has 100 friends, etc), so I am not sure if this is not a performance issue, although my computer easily exceeds the specs, and I have never had this before.
Maybe noteworthy, my game runs on SSD, and I never defrag (they say you should not), but it is hardly 40% full.
However, thank you very much indeed, it helped, and I am probably going to contact the Bug Report Forum from now on.
Rinkils