The bug

When a chunk is unloaded and reloaded, villagers will reset their iron golem respawn timer, causing a new iron golem to spawn in less than 30 seconds after the last golem was spawned.

This causes golems to be able to spawn in very large quantities even if there are already a lot more golems present nearby, potentially causing performance issues.

To reproduce

1. Download and load the attached world: 2020-03-14_00-02-27_Iron golem spawning.zip (version 20w11a or newer)
2. Toggle the lever
3. An iron golem will spawn and you get teleported 10,000 blocks away
4. Press the button
5. Wait a few seconds
   → Another golem is spawned

Videos

As far as I'm aware, this was originally showcased by pags5zonda in this video: https://www.youtube.com/watch?v=6SxxmhF8Kk4

Here's a short video of me reproducing the issue: Minecraft 20w11a - Singleplayer 2020-03-13 23-57-07_Trim.mp4