it's so good 😄 I hope they improve generation speed in newer builds tho, that's the only thing that's a bit annoying atm

I'm using the Simply Optimized modpack plus this mod, you can see my GPU and stuff in the screenshot. my actual render distance is 4 and then adding the 128 chunks from the mod, it gets me around 140-160 fps

You can actually bump it up to 256 or 512 without a huge perf drain depending on your setup, distant horizon doesn't use the normal game renderer... You do give up shaders for it...

It's a setting you can change. The default is a "quick generation" mode which looks as similar to the world seed/settings as it can without taking the performance hit of actually generating it. But it can either be changed to generate the chunks genuinely out to the render distance (which would be lag hell for 128 chunks) or to only render already generated chunks (like you suspect). Only the latter works in multiplayer though.