The shortness of a thing is an aspect of its length. Your comment makes no sense. Five times shorter means that you can play Mirage five times in the same time it takes to play Valhalla once. I'm pedantic as hell but this comes off to me as fake pedantry.
Yeah, I'm being quite pedantic, but it is genuine. It's a common language mistake that frustrates me.
Don't try to multiply a relative descriptor when you can use hard numbers (i.e. it's 20 hours shorter) or the appropriate relative measurement (i.e. it's 20% as long). It's both improper English and less clear (maybe "2 times harder to understand"?) to do so.