Adding the borderless mod cuts performance in half...

[cpm9]

Big performance hit ~140 fps down to around 60 fps

Using borderless mining 1.1.1-1.18
fabric-api 44.0
sodium 0.4.0 alpha 5

before adding borderless

after adding it

You can see a big increase in the proportion of the updateDisplay (light blue) part of the pie chart

[comp500]

Unfortunately it's unlikely that I'll be able to do anything about this - windowed borderless is fundamentally slower since it has to pass through the compositor.

Convincing Windows to enable Fullscreen Optimisations (a middle-ground between exclusive and windowed borderless) might help, and it's possible that the changes in #19 will do that. But I don't really know; fullscreen/borderless window detection is built on a bunch of broken heuristics that vary between graphics drivers, where there's often no way to get it to do what you want.

[cpm9]

Largely improved in newer versions, closing ticket