as title suggests, if one uses a custom generator settings, instead of the default `"settings": "minecraft:overworld"`, and doesn't list any structures, then the game will generate structures in their naturally spawning biomes at an extreme rate. e.g. pyramids in deserts. If this happens in the nether biomes, due to the bastion remnants spawning with a large amount of mobs, extreme lag will happen. 

just the overworld - no structures.json

https://cdn.discordapp.com/attachments/514558568624685088/713459377608327238/2020-05-22_19.32.37.png
https://cdn.discordapp.com/attachments/514558568624685088/713490134523183217/2020-05-22_21.34.10.png

What should happen is that any structures not listed should not generate at all.

p.s. this doesn't happen in superflat worlds.