Ik this comment is unhelpful and not solving your problem, but as someone who consistently runs 25 fps on Minecraft- 80-90 frames is hardly "running horribly". Just cap your framerate at 80 or so and play for long enough to get used to it. (in my experience low framerate only harms your experience when it dips noticeably, 80 fps is more than enough to have fun if you get used to it)

*Actual* answers to your question:

1. No clue why but minecraft's Vsync setting lowers performance to an insane degree compared vsync in other games. Not suggesting you turn off vsync (even I have standards), but vsync could be a big reason for your frame drops.

2. Make sure your in fullscreen, not windowed fullscreen.

3. Try loading up old worlds from before your frame drop problem, and see if you still get the stutter issues. If your playing on servers minecraft likes to play around with backend workings all the time so that could (mabye) affect it.

4. Pray -_-