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).