The MSPT (Milliseconds per Tick) increases if the player is above water and y>190. How to recreate:
-create new creative world
-fly above ocean and wait for chunks to load (I used renderdistance 16)
-go to y=180 and wait till MSPT are stable
-go to y=200 and wait till MSPT are stable
There should be a huge difference between those values (see images). Above land this does not occur.
This height is nearly exaclty the 128 blocks (instant-despawn-range for mobs) above the water, maybe this is where the lag comes from. I hope this is not a duplicate (I couldn't find a bug report describing this problem).