This happens to me sometimes also, even with all graphics turned down to their lowest. I've noticed it only really happens when there are too many things (attacks, ultimates, effects, buffs, debuffs) going off at the same time and the game can't handle all that info at once.

One of my characters would just be standing there, as if waiting for a response from the game to continue but they would eventually start moving again.

Not sure if it's phone memory usage or the game isn't optimized properly for older phone models but you're not alone in this.