@hyarion I think it has been worse since the new winter backgrounds, its always been there going to quite a long time on all the backgrounds but it was annoying but tolerable. It's gotten a lot worse recently, so either that or version changes to IOS or the game itself. My normal iPhone 6S has the same problems as my 6S plus and 7 Plus though.
My iPad mini 2 drops framerate but it doesnt feel like the input is affected the same as the iPhone, on the iPhone when it starts getting bad the input has issues too so it responds sluggishly. On the iPad when it drops frame rate it still feels pretty normal and I can play though it decently.
Maybe some options to be able to disable backgrounds or the dirt kicking up could help. I'm not running the HD graphics either, I reinstalled and canceled it when it attempted, but it seems to be identical with or without the HD graphics.