Lag,I suspect from multiple(to many) mobs spawning from more than one mob spawner in the area, I say this because I played with no instance of dicernible lag when there was one mob spawner( zombie,skeleton or spider) in the area spittin out 4 to5 mobs at a time,but when i go to an area where from count there are 7 to 9 mobs coming at me endlessly, the closer i get to the area of the spawning the increase of lag occurs,reducing fps to 0-1
this has happen on 2 worlds created
I feel its multiple mob spawners in close proximity to each other,and if so is that intended ?
or,which i highly doubt, one mob spawner spittin out endless mobs
Also when defeated by said mob(s) when I attempt to go back to the area just above the mob spawner up top in the open world ,I get to the area of where the mob spawner is below, that is where I experience the lag(reduced fps 0-1)

Also I feel the lag(fps reduced) is from dropped inventory after the mobs dispatched me, upon re spawning go back to area to collect dropped inventory only to find fps drops significantly ,only increases after when dropped inventory disappears after the period of time for said inventory to disappear