TLDR; 256x textures + any shader (on Lite setting) + populated areas (plain animal farms) = below 60 FPS.
What bothers me is that in those areas, my CPU and GPU seem under-utilized, where GPU sits at around 30-40% usage, and not a single CPU core is above 50%. There is obviously a bottleneck somewhere, but I can't figure it out. In fact, when I do get high FPS, both CPU and GPU areused more. But in some areas it's as if something chokes everything, and usage falls down (aka I'm not hitting CPU/GPU "overused").
I tried a bunch of popular shaders, from Lite to High, doesn't make a difference (aside from more GPU usage in some cases).
Without 256x, FPS goes to around 80 FPS in those areas, which also seems a bit low.
I'm playing vanilla SSP (OptiFine is the only mod). I prefer ProjectLUMA or Sildurs Vibrant/Standard Lite.