I created a Realm not very long after it was out for the XB1. I had a few people join and we began working. We found 2 spawners, one zombie and one skellies, under the base we built near spawn. We built traps and pretty soon, the lag became so bad that we simply could not play. It was physically nauseating. It happened on the way down the stairs (receiving damage and dying) and in the overworld when above the spot where they were.
We wasted time and MY MONEY because I had to replace that world with another just to get people to play again.
We have not yet found a spawner in this new world, but we aren't too anxious to try, either. Please make sure this gets fixed.