Constant screen backlighting and CPU use will eat the battery. The backlighting likely uses more power than anything on your phone.
Most who use their phone for navigation, use a power cord.
Even using GPS in Apple Maps drains battery too, but I have no idea if it's any better than Google Maps.
I used Apple Maps today, it drains much less battery.
Lock screen integration and sleeping the screen unless prompted for a turn most likely helps with this.
My screen is on the whole time.