After a complete redo of website in over a decade, YouTube introduces one of the most wanted feature on Mobile apps. The playback speed controls for a YouTube video is available on web for years but it was always in demand for mobile Apps. Finally, YouTube has introduced the video playback speed control feature on iOS and Android YouTube apps.
With this new feature, users can slow down or fast forward the speed of any video. Users can watch a video on the slowest rate of 0.25x (quarter speed) or the highest at 2.00x (double Speed). To slow down the speed or fast forward it on mobile was very challenging, the feature is called time stretching. It essentially means, ir-respective of the speed of a video the voice does not get compromised.
YouTube has explained the full algorithm on their blog at https://youtube-eng.googleblog.com/2017/09/variable-speed-playback-on-mobile.html