Download an external player that lets you control the speed, like BSplayer. 

Then in the youtube app when you get to the video you want to watch click the Share button and then select the player you want to use.  The external player will open and it will play the video you want, and you can control the speed in it.  


Tip: you can also use this method to skip ads,  if you share the video to a different player you can watch it right away without having to wait if there's an ad. 

Edit: I mentioned VLC and Dice Player as other options to control video speed,   but looks like I can't share a video from the youtube app to play with them, at least on my phone