By fast leaves I mean the non-transparent leaf texture. There is a mod that implements this, but it seems somewhat abandoned. From what i can see, it turns leaf blocks non-transparent, if it is surrounded by five other blocks on any side. I don't know how much this would improve performance.

I actually looked at it again, and i don't think it works like this issue's title says. Maybe it works by checking if the block i touching air?

Don't we already do this with one of ours? https://github.com/FxMorin/MoreCulling/wiki/Leaves-culling#default
Also that mods description is false. We also inject into sodium chunk builder for our culling optimizations

Oh, sorry didn't notice the wiki page and didn't know what the options meant. On another note, you could make a tooltip in the settings for the culling options explaining the different settings. I will close this issue.