I upgraded  a world to 1.18. I was generating new chunks and filling out maps, and it was running fine, but I've found a spot in an ocean biome that partly freezes the game. I can still move, but no more chunks will load, I can't interact with anything, commands won't function, and when I save and quit it gets stuck loading so I have to force quit. It's the same spot every time. I have 8GB of ram allocated, and I tried optimizing the world and it didn't help. I don't know if this is a world generation issue or something specific to that area.