it's the black gradient on the 4 corners of the screen that minecraft uses when you turn on fancy graphics

Can I use the vignette disabler that sodium provides?

@spidernh

Phosphor improves chunk lighting performance, while lithium improves server performance but can it improve Singleplayer performance as that is running an integrated server (the thing that makes LAN connection easier if you switch over to LAN). Just to clarify I don't know exactly know what lithium really does but that's just a broad overview of what it does.