When in the the overworld I always get 800+ fps but while in the nether I get 30-40 fps (see picture 1). Also when mining anything it takes 3-4 seconds to be mined and drop. This only happens the first time i entered the nether, after saving and rejoining the lag is almost gone. I still get 70 - 110 fps, not nearly as much as in the overworld. Its not my PC as it is completely overkill for MC.

EDIT: after 3rd rejoin my fps is high again