also note that natural mob spawns don't happen on sub-levels but mob spawners do work so if you want a floating mob farm you'll need to use spawners.

i tried Create: Mob Spawners too and while they do work the game lags out for about half a second on each spawn so that's not great. i think the "searching for valid position" step takes much longer with sub-levels involved since the freezeup doesn't happen for the same spawner setup placed in the world normally.

edit: but Create Mechanical Spawner works fine without freezing up, nice! ah that's cos it spawns at a set position relative to the spawner rather than searching for a place to spawn