I totally agree that people do use a fast metabolism as an excuse of not being able to put weight on. Where the fact is they simple dont eat enough.
Everybody can put weight on! But to say "there is no such thing as a fast metabolism" is just not right!! Also we are not cars!
One's metabolism speed does vary from person to person! There for logic dictates that some have a fast metabolism and some have a slow metabolism.
I would say governing factors of a persons metabolism speed come down to a few things, such as genetics, amount of LBM , what sex they are and there age. (might have missed something

)