Hey,
So the game plays totally ok in combat but the menus can become really slow sometimes, I’ve never had any crash but it’s so slow that I have to restart the app.
I don’t know how’s the performances on the 5C or 5S (Better on the latter I suppose) I hope you guys finds a solution. 
Performance on iphone 5c is alright but very taxing on the device, often leading to overheating in two of my customer’s phones.
I have the same performance issues on a brand new Huawei G Play Mini: it starts out fin but, after a while, it overheats the phone and performance plummets.
I’d say they’re all the fancy particle effects, but that’s an educated guess.
So I’ve done some minor testing.
Leaving the game at rest, the phone still gains heat. Enough to be noticeable.
Playing around in the menus for 30 minutes net a heat that was uncomfortable.
Playing the game for 1 hour straight and the heat from the phone seemed a dangerous level, so much that it slightly burned my hand if I held the iphone 3c.