+1 just adding that after flashing and booting including ART buildup and media scanner running that can happen even on charger, perhaps not on this device but my OP2 and my Nexus 6 can discharger even on charger during these processes with a large nandroid performed beforehand.That's pretty normal I would say if you were using the phone for those entire 15 minutes.
depends a bit on the charger cable at times. all those processes are very power consuming.