Your playback quality is based on your internet connection speed which falls within one of the three categories below.
Sometimes, even where you have a broadband internet connection, your connection speed could still be low based on a number of factors.
A fair rule of the thumb is, if you encounter a BUFFERING PROBLEM (i.e where the video plays and stops, plays and stops, again and again) then you are better off changing your chosen speed to a lower one.