Yes we can slow it down! While you are learning to play a piece of music with your Youtube backup band you can adjust the playback speed to slow, slower and slowest.
For the music that plays from a YouTube Video look at the bottom bar of the YouTube display screen. There is a place for you to play and stop. There is also a volume and time remaining display. Move over to the right, and see that? There is also a Settings cog.
Click on the Settings cog. Now you should be able to see several options that include playback speed. You guessed right. Click on Playback speed.
How did we ever survive with out the technologies of Youtube? Don't answer that; some people haven't. But here it is in it's amazing glory. Play whatever speed you are comfortable with when learning, and your backup Youtube band will play along.
It's as easy as that!
I discovered this feature when listening to podcasts and found that I could listen to a podcast way more quickly when it was playing at 1.25 or 1.5.
to this feature!! 
It works well both ways