Something definitely seems out of whack. On my BR2205 2300KV motor, it will do a 6x4 prop and generate 550g of thrust at 14A. That's 3.6 minutes of full throttle on an 850mAh. 70% throttle is 6.6A or 7.7 minutes. I know the slot creates some extra drag on the prop not not twice the power. A 6x3 prop drops max power down to 12A and only 520g of thrust, so that could help your situation but not by much. Seems like the motor is pulling an abnormal amount of power to run. Did you check to make sure the motor bell isn't rubbing against something and slowing it down, or maybe the center hole in the firewall isn't wide enough for the motor shaft to not rub and pull extra power?