I have noticed that Warframe sucks battery when it is in suspended mode. i.e. the iPad is locked or I have swiped out of the game and are using another app.
I stopped playing Warframe at 70% battery, locked my device and when I picked it up again a couple of hours later it was at 34% battery.
I've never had my ipad suck 40% in a couple of hours. Usually it takes a good few days for the battery to drain that much.