Low FPS when I put grass on – Carnivores: Dinosaur Hunter Review

I love this game, everything about it but there is one problem. When I turn on grass, primarily in medium and high, my game starts to lag and the FPS is like approximately 10 or 12 frames per second and it is really lagging. What do I do? I am on the iPad mini 2. Please respond Developer. Thank you.
Review by Needs one more mode. on Carnivores: Dinosaur Hunter.