@jliu670 The bottleneck is the game engine: you're giving it more to do than it can handle properly. There's no fix for this other than to reduce the load. One thing you can try is playing at a lower resolution, although I'm not sure how much that will help. If it did, the conclusion might be that you'd need to play this household at something closer to 1080p rather than 4k.
You can also try Simp4Sims's Graphics Rules Overhaul, that is if it's been updated for the DX11 patch. Make sure you're downloading one that was posted after that patch release; the older version caused the game to stop working entirely. The idea of this file is that among other things, it lowers the number of times the game switches between the high and low LOD versions of objects, and that switching itself can be a significant source of lag.
Having said that, it's reasonably well-established at this point that Sims 4 can't handle large lots with thousands of objects. It's clearly a game issue, not a system issue, and until and unless the developers fix it, there's not much you can do on your end.
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