How long have you been playing that save? I ask this because when I tested that I was was getting FPS in the 30's. When I went back to my current save, my main farm is 159 and I also have the BSL dairy just north of that. My FPS was maxed out at 60, weird cause I TP'ed to that grass field and my FPS stayed around 60. Only real difference between my current save and the public one is mine runs thundr's RCR.
So I asked about your save maybe it's doing a cache thing similar to after the game updates. If you bought it on steam maybe try verifying files. I'm going to have to keep an eye on it.