At the start of a new world that world is getting generated, lights calculated and long distance lod generated which takes a lot of cpu time. When then that is all done it generally uses less cpu, however mods, sprinklers, large farms, lots of animals, etc can all require a lot of cpu too.

I'm a total layman when it comes to game design, so forgive my ignorance on the matter, but couldn't there be a loading screen while the world is generated instead of having it generated while you're playing? Or does it not work like that?

Originally posted by Taijitu Taikyokuzu:

I'm a total layman when it comes to game design, so forgive my ignorance on the matter, but couldn't there be a loading screen while the world is generated instead of having it generated while you're playing? Or does it not work like that?

The seed is what causes what to generate where. Just like in Minecraft. Even if something is too far away to render / generate for the first time, or even if it takes hours for you to get to that specific chunk/cell/etc. The seed will always have the same things generate in the exact same locations. (There are exceptions to this rule, such as a jelly on the side of tree leaves, or a bug appearing over a flower, which can generate even if you're next to said flower and just AFK there.)