Bug can cause immense lag upon switching to update 21w05a and while using this update. I have no idea what causes the bug, but the entire game freezes and will not load properly from then on, having only stutters of loads every few minutes. My best guess of the cause would be an error in the way the game is saved, because it's first occurrence happened directly after loading an old hardcore world into the newest update. After trying to close the world the game also slows to a halt while saving. After attempting to load the world a second time without backing up it functioned normally, but the bug occurred again ~15 minutes later with no noticeable in-game cause.