The C&C Preview Datapack can sometimes create biomes with the ID "minecraft:null" or turn biomes that aren't oceans into "minecraft:ocean" biomes. The corruption creates intense lag while crossing the chunk borders.

How to recreate:

1- Create a world with the C&C Preview datapack.

2- Go around and load chunks, may be random but this happened to me the most when I had ocean biomes loaded

3- Biomes with wrong ID's will generate and the corrupted ID's will spread to already loaded biomes.

4- Once the corrupted biomes are unloaded the bug stops and world generates as intended.