I have a similar crashing issue that appears to be related to the number of Chickens/Cows/villagers are loaded in memory...
when I kill off animals, the issue improves
when I bring more animals back, the issue returns
when I go far enough away from my base that the animals are removed from memory the game stabilizes.
Happens across multiple worlds and across multiple iOS devices (tried ipad mini and iphone 6... no perceived difference between the 2 devices... the crash happens before the game slows down.)
This should be fixed in 1.6.2.