Sims 4 Massive FPS Drops

@Gameshark144  This is definitely a game engine issue.  There are a few different causes that I know of, and probably more I have no idea about, and I don't think they can be fully corrected.  Having said that, there may be a few steps you can take to at least mitigate the effects.

Please do the following in a clean user folder.  Move your entire Sims 4 folder out of Documents\Electronic Arts and onto your desktop, and the game will spawn a new one the next time it launches.  Don't add anything to the new folder yet; just test in a new save, in one of the base game worlds, with no custom content or mods.

First of all, tell Windows to run Sims 4 in performance mode.  Hit Windows key-i, select System > Display > Graphics Settings, click Browse, and add TS4_x64.exe.  Then click Options and select "High performance."

Please try playing on ultra graphics settings at your monitor's native resolution and see how it goes.  If you're willing, I'd be interested to see how fps behaves with no cap, at least for a few minutes: it would naturally fluctuate somewhat, but framerates should be much higher at all times.

If you see the same significant fps drops, try playing in a clean boot:

https://www.windowscentral.com/how-clean-boot-windows-10

When you reboot, go through the Task Manager's background processes list and shut down anything that's not absolutely necessary.  Even though the entire point of a clean boot is to make that step redundant, anything from MSI Afterburner to RGB software might still be running, so you'll need to check.