I've been experiencing FPS lag, while the F3 screen showed I have 238/104 FPS. So, that's weird to start with. I tried updating my drivers, optifine, using special GPU, etc. All the usual stuff and nothing worked.
Then I thought: Hey, I'm playing Minecraft on my laptop, but with an external monitor, let's try and play it on my laptop screen instead.
So I went to [settings] -> [display] -> [Extend these displays] to turn on my laptop screen, because I was playing on my big screen with the laptop screen off. And POOF! It starts running smoothly again.
I'm not even playing it on my smaller laptop screen, still on the same monitor. Just changing the display settings from only showing my monitor to showing both the monitor and the laptop fixed the problem. It's a waste of energy to have two screens on, but that's how it is now...