How to slow down the youtube video playback speed?

Absolutely, you can slow down the streaming speed of YouTube video by clicking the Gear option visible at the right bottom corner of the video. Click on it and chose 'Speed' which shows the following options 0.25, 0.5, 0.75, 1.0 (Normal), 1.25, 1.5, 2.

For example, if you click on 0.5 option, the playback speed will be reduced to half. If you chose, 1.5, the speed will be increased to 50% more than the original playback speed.