Those take place during worldgen though, which is a one-time event that occurs before gameplay. Where most players actually experience CPU problems (and it matters) is during gameplay. The world is effectively frozen after world creation (though I think the latest version does allow for events to still go on), but what has historically been the source of performance problems is dwarves and animals pathing, large numbers of items, and water/lava flow. Even though you can generate these huge regions, the average fortress is built on a pretty small map.