This is absolutely ridiculous. A music player in "offline" mode and with background refresh turned off has slashed 80% of the battery on my new iPhone 7 (new in January) within 6 hours while on wifi. Phone gets disturbingly hot as well. Been doing this for a week or two now. An MP3 player from the early 2000s can perform better than that. Figure it out.
Also I should mention that it's made my phone so hot that it just shuts down even though it's not out of battery. Don't know if anyone else has had that experience. I pretty much just try not to use spotify on my phone anymore which is a shame, because it's great.