I have a big cow and sheep farm, there is a lot of them. They are under the ground, however if I look at them through grass blocks, fps instantly drops to 30. (Optifine + Complementary shaders go fine in 60 FPS VSync)

Shaders might still be impacted, or it might be too close to the wall. I don't test with Optifine shaders(I play with Sodium+Iris), so maybe Optifines shaders just react differently in that case.

Closing as stale.