extended world heights lag

when using extended world heights the game is bit jumpy no idea why.

i have added the pack i made to test the min needed to remake the bug

it is saying this in log Can't keep up! Is the server overloaded? Running 2008ms or 40 ticks behind

the time and ticks vary 

the pig picture is a device to see the lag staying still and the pig bit is me just messing around its not needed

the clock and piston stops when the game lags

do it now in no datapacks  cant previde metrics as it cant work due to bug with f3