Processing power most likely is not the problem at all. It might be the drift rate of those gyros, which appears to be very high. I used one AH app on my iphone for a short 45 minute flight, at the end of that sector (back at the gate) it was off by more than 30°.
I agree about the GPS drain, but you can of course offset that by using an external GPS mouse with bluetooth LE connection. But the biggest drain on the battery is usually the display anyway.