Same problem here. My sense is that this is a problem with iOS, not hearthstone. Or maybe a bit of both. I don’t want to get all conspiratorial, but apple have a new mini coming out this week and need people to buy it. Suddenly performance of mini goes through the floor. My other games are performing like absolute crap too, so I don’t think it’s just hearthstone.
Aside from that, game is basically unplayable for me at present. Constant crashes when matchmaking and game can’t cope with gold cards and crashes after every single game.
Running iPad mini, iOS 12.1.4. Problem did not exist before 12.1.4, but I appreciate the hs update and the iOS update came on the same day.