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 !!