There are many reports to the YouTube customer support account on Twitter complaining about battery drain when running the app on iOS 11. The company is apparently ‘actively working to fix this’.
The YouTube app appears to have some bugs on iOS 11, affecting all iPhones and iPads, causing the devices to run very warm when watching videos.
This is happening on my iPad Pro 10.5”, but it doesn’t get warm at all — the battery drain is through the roof though. Using YouTube through Safari solves this problem though.