It's obsolete.

there is already a "payback speed" option in the settings of a youtube video. and it has all the SAME options as well.

What's the point if it's the same speed as on YT?

Doesn't work on Android at all.

Why doesn't it work?

YouTube Video Player Speed play in whatever speed you want. 2x 3x 4x 5x and 0.01 speeds are available to play. Better...

Only goes up to 2, same as default player.