if you set world spawn to 0,0 and stand at 0,0 in an empty world you may have INTENSE lag!

1: create a world in superflat custom only one layer air.
2: set spawn at 0,0 and go to 0,0
3: build something around that area so you load everysingle bit of the chunks around it (I used mc edit and drew a square 32x32 around the middle)
4: Intense lag. but FPS doesn't seem to drop...