>The server was prepared with a 100k x 100k block pre-generated world. Our custom plugin distributed new players to the least-occupied region.
Minecraft chunk generation is notoriously though, and even a few players generating new chunks will bring the TPS down to an unplayable state very fast.
This is true to an extent, yes. Chunk generation can be real tough. That said, chunk generation has been re-written in Paper, so it's substantially faster than vanilla Minecraft.
When we did a large scale real player test on a much earlier build of Folia we had ~327 players at peak and we did not generate chunks because we specifically wanted to see how it ran. We didn't max out at ~327, we just didn't have more joins than that.
The project improved a bunch already so that number without chunk generation is very doable to beat if you didn't pre-gen any chunks.
Generally it's free performance to pre-gen your world though. Highly recommended for regular players.
And that's knowing a big part of the chunk generation already happens off the main thread.
That's not happeneing, it says "pre-generated world"