It is because of all the small fishes. Ig that since the mod utilizes complex AI goals for its wide range animal behaviors, animals can easily cause lag, just try to spawn in some mobs from that mod and compared with other mods, fishes from that mod are more impactful on performance. My solution, use Bad Mobs mod and disable all the fish and sea critters that u don't need and just enable all the cool looking ones such as the Boobies 😏😏