Every few minutes, hundreds of aquatic mobs (400+ cod is typical) will simultaneously spawn. (This can be observed by tracking entity population on the scoreboard with a tick function.) The mobs will then gradually despawn until getting below a certain threshold, at which point the cycle repeats. It appears that the mob cap for aquatic mobs is not respected within a given spawn cycle. This can cause severe server lag; a debug log taken during such a spike shows 36% of tick time was spent on cod AI alone.