Dokkan Battle is a 2D game.

Phones tries to use the cpu to process about 2D graphics

So, the cpu is very busy.

Please redirect all gaming needs to the GPU, tick the box next to :

Go to Settings > Developer Options > Advanced > "Force GPU rendering"

If you don't see developer options in your settings page simply go to

Settings > About > Software Information > More

and tap where it says "build number" 7 times, you don't need to be rooted !!