iPhone battery drain

I recently noticed - possibly related to the iOS 11 update - that my iPhone 6S loses power significantly more rapidly with Pokemon Go open than it used to. It's so bad at times that I can literally watch the percentage drop. PoGo was always a battery hog, but it's gotten to the point where I need to charge my phone constantly if I want to play the game throughout the day. Is anyone having a similar experience and do you have any tips? I do keep Battery Saver mode on and have the sound turned off.