I've learned to like 2x video playback on my computer, but the official YouTube iOS app doesn't suggest that option.
What ways are there to watch youtube videos at custom speeds on iPad / iPhone?
Speedeo just came out and allows you to control the speed in 0.25 increments. It's pretty sweet I've been using it for the past week or two.
Also good because the history is easy to access so you can close and reopen videos easily.
According to this question and answers - YouTube at double speed on mobile - there are not many good options.
The Swift Player app (http://apple.co/1OQpNDW) can, but it is not availabe in the US.
The Play Tube Free from VietMobile can also (http://apple.co/23uwcMy) but it also is not available in the US.
Supposedly you can switch to the desktop view and speed it up, but I haven't gotten that to work.
This can now be done with the latest version of the YouTube Mobile App
"What's New in Version 12.33
• Adjust video playback speed with new player controls"
I made an action extension for iOS Safari that does just that. For those unaware, action extension basically adds button to Safari action sheet that allows you to adjust video playback speed. Works on both iPhone and iPad. It works with YouTube and virtually any other HTML5 video player, in Safari itself, rather then having to open different app. https://itunes.apple.com/app/id1205188880
You will need an app to help since YouTube doesn't allow this in 2017 on safari and native browsers.