I have iPhone 6 as well. Two batteries worth of flight time and I'd drain almost 70 percent of my battery. My phone is over 2 years old (almost 3 years) and works fine otherwise. Just think it's an old battery. Upgraded to iPad mini 4 last week (wifi only) and two batteries of flight drained 7 percent of the battery! Good luck, but I'd consider upgrading the mobile device for your RC.
GO app draw battery, that's clear. But if you use OTG cable between phone/tab and RC, where you insert the "OTG" adapter has an big effect on power supply. Thepower supply will be connected together (tab and RC) by the cable. The side with the OTG cable will become the host and will try to charge the device on the other side of the cable.
Measuring the current in the USB cable give for me:
1) Samsung S6 phone with OTG adapter, RC with normal USB cable, current is 1.0 A, yes 1A
2) Samsung S6 phone with normal USB cable, RC with OTG adapter, current is 0.17 A
So you can guess which way I am using now, 2).
I tried also the 2) configuration with a iPad Mini4 and got 0.10 A. Didn't want to try the opposite way.
The phone/tablet is made to power with high current a USB device where the RC can only deliver small current to an external device. The RC can run 2.5 h so at 0.17 A at 5V it is about 2 Wh. The battery in the RC is 3.7 V rated 3 Ah, so it has 11 Wh.
So we are talking of a reduction 20%. So we can expect a run time of the RC of at least 2h. This is more than 4 flights using the battery in full. For all the benefit of the cable connection, it is not a problem IMO.
For the case 1), your RC battery will be nicely charged by the phone/tablet but then your phone battery will go down quickly. In my case of S6, it is 3.7 V 2.5 Ah = 9 Wh, having 1A at 5V on USB, it would be 5 W, so it will have huge impact on the phone autonomy. At some point, the RC will be fully charged but the RC need about 4.4 W, so even with RC charged, there will be still a high current I would guess (didn't want to test and stress my phone/RC for the sake of verifying a theory).