Autosaving now causes extreme and gameplay-impacting lag

In 21w42a, whenever the world autosaves, there's a huge lag spike compared to auto-saving in 21w41a. This issue also causes legitimate gameplay issues, as mobs are able to attack the player some time before the world unfreezes, resulting in unfair deaths. I've noticed that it gets even laggier if you've mined some blocks and moved away from spawn as well.

Steps to reproduce:
1. Open a world.
2. Punch some trees and move away from spawn.
3. Wait for the autosave.
Expected behavior: The game will autosave as usual, with little to no lag.
What actually happens: The game freezes for a few second, and mobs can attack the player before the game unfreezes.