Too many mobs make moving impossible because of lag

Steps to reproduce:

1. Create a zombie grinder of any sort
2. Stand near the grinder and let zombies spawn for a long time

After a while, there are so many zombies that it's impossible to move or to attack the zombies to reduce the number of hostile mobs. Also, the mobs never disappear on their own, rendering the world unplayable.

This is a duplicate of MCPE-15940 which was incorrectly marked as resolved, as the issue still exists.