The mob AI must have been affected in some way over one of the recent updates, as hostile mobs do not react to a player walking by very quickly. I have noticed it takes a while before the mob will start coming towards you, and it will also restart whenever they are hit by the player. This is especially a problem with Spiders, who will not come towards a player even when there is no block or obstruction in the way. Also, the spawn rates are very low, as I have noticed whenever I go outside at night, there is on average 2 mobs outside. This makes Survival far less challenging. This needs to be fixed in the next update.