Playing in 20w17a with the caves and cliffs datapack in singleplayer. Going near aquifers causes the biome to become null and it persists everywhere after initially becoming null. Every time you cross a chunk after your biome becomes null a very prominent lag spike happens. The null biome can be fixed by restarting the world when not in or near an aquifer. I have tested this with multiple aquifers and they all cause the biome type to become null and then lag spikes will happen every time you move between chunks until reloading the world when away from an aquifer.