Is it an older phone? Water Pulse used to kill the frame rate on my iPhone 5, which was the last iPhone to have a 32 bit processor compared with the next generation at 64 bit. When I upgraded to an iPhone SE, a lot of my previous PokemonGo problems disappeared.
If it is an older phone, then other than upgrading the best you can do is note which move animations slow down your frame rate and avoid using them. You can try sending a Report Critical Issues message to Niantic.
