Have you started to disable/uninstall packs?
Yesterday I disabled the lovestruck expansion pack in my save. The reason was because I noticed that my loading times in my save started to increase more and more to the point where I had to wait a very long time to load a household and after loading a lot the game could take a minute until sims were allowed to start moving.
I'm not sure if lovestruck was the issue, the thing is that my save file is a bit extreme, it's an old save file from 2019 which I have filled with different gallery sims over the years, to the point that it contains 1500 sims (I found this out with MC Command Center get_pop). So the save is pretty demanding already.
Disabling mods did not get rid of the loading issue, reinstalling the game did not get rid of the issue, however I did notice that if I loaded an older save (around maybe april, the loading time problem was gone), but as soon as I went into CAS, set some settings like turn-on and turn-offs, the loading time issue was back. So I started wondering why that save was fine but turned bad as soon as I tried to play and then I remembered. It was around April where I had re-enabled lovestruck in my save.
Disabling lovestruck on the save around april got rid of the loading issue, so my theory is for now, that at least in my save, I can't have too many packs anymore. Lovestruck might be exceptionally demanding because one thing that it for example does is that it assigns 50 turn-on and turn-offs to all sims in the save, which in my case is 50 x 1500 = 75 000 turn-on and turn-offs. And yes, I'm aware mods could be the issue too, but I tried disabling them, no improvement to loading times, disabling lovestruck however, worked.
So I thought I could share this here, and maybe this could be a warning if anyone has any save similar to me, I'm afraid that this game might have reached it's limits, at least for larger save files like mine. I'm not sure I will have the strength to start over again.