I know the slow motion thing has been posted before (when there's to many mobs or you lag mobs go in "slow motion mode" to stop lag from happening) however, I found out that when this happens it completely stops mobs from spawning in my world. I had maybe 2-3 zombies spawn over 4 minecraft days, and I was in a cave most of the day. I also had opened a creative superflat world and set it to night, it spawned about 30-40 mobs than everything went slow-motion and it completely stopped more mobs from spawning. I understand this maybe be a "feature" as it stops lag, however, I can't get mobs to spawn in my world because it stops them from spawning.