When I load a void flat world (generated in vanilla 1.16.2) onto the server using WorldCreator.name("test").create(); (Everything works fine if the world is the default world) and teleport a player to spawn, everything is fine. As soon as the player comes close to chunks that haven't been generated yet, their client log gets spammed with "Received invalid biome id: -1" and they will freeze to 0fps for a while until they recover and are fine again until coming near a chunk again that has not been generated. In single player i can freely walk around the world and the regions i have been in single player and on a vanilla server are completely lag-free when it has been put on the spigot server. It is the same setup i have used for years and has never been a problem prior to 1.16