I started the server in 15w46a a short while ago and mob rate was fine, have upgraded server to 15w47c and spawn rate of hostile mobs has dropped dramatically, have checked with host and they believe its a snapshot issue as the server appears fine. Have restarted, stopped and started and has not affected the issue. Also I have run the server for over a week now, have a house in a swamp and have not seen a single slime, there is a witch hut nearby and no witches spawn either.