I'm on iOS 10.3.3 on my 6s+ and YouTube is killing the battery. Was between 40-50% and 15 mins later, it went down to 2% and I think this started with the recent app update.
Guess I'll have to wait for the app to be updated.
Were the videos HD? You have to pick a lower resolution. No app update can fix it, if you want to play HD videos. iOS devices only have hardware support for mpeg4, not vp9.
You have to pick non HD videos to get mpeg4 format videos.