There's no such thing as a "fast metabolism." How much energy you burn is strictly a function of how much energy your body needs to keep itself at ~98.6 degrees and how much work your body has to do.
Right. So two furnaces run at 98.6. Is it not possible that one is more efficient at maintaining that temp (slow metabolism) versus one that is inefficient(fast metabolism). That’s why people suggest finding your basal metabolic rate (BMR) and from there everything is CI<CO.