One minute lockup between world gen reaching 100% and entering the world

When generating a new world in single player, the client locks up for at a minute or so after the world generation screen has reached 100% and before the player is allowed to enter the world.

[20:39:52] [Render thread/INFO]: Preparing spawn area: 84%
[20:39:53] [Render thread/INFO]: Preparing spawn area: 90%
[20:39:53] [Render thread/INFO]: Preparing spawn area: 95%
[20:39:53] [Server thread/INFO]: Changing view distance to 12, from 10
[20:39:53] [Server thread/INFO]: Changing simulation distance to 12, from 0
[20:39:53] [Render thread/INFO]: Time elapsed: 11259 ms
[20:41:16] [Server thread/INFO]: Auzathoth[local:E:afe9cf98] logged in with entity id 178 at (3.5, 112.0, -1.5)
[20:41:16] [Server thread/INFO]: Auzathoth joined the game
[20:41:17] [Render thread/INFO]: Loaded 0 advancements
[20:41:22] [Render thread/INFO]: [CHAT] Seed: [659839236]