I made a fish tank and lured a sea turtle into it with sea grass.  After doing this I noticed the entire realm would lock up for a moment every 10 seconds or so.  You could tell it was happening because the fish would stop swimming, blocks would not break / reappear, players would not hear themselves eat.. etc.  I believe this may be related to the Sea turtle trying to swim back to its home beach but unable to.  In my case I bred the turtle in a castle with some sand and water put in an area.  then lured it maybe 20 blocks away to a glass encased water area with a bubble column in the middle (maybe 5x5x5 blocks of water).  upon killing the turtle the lag seems to have gone away.