There is no such thing as a fast/slow metabolism by the way. Ok ok, there are exceptions to every rule where hypometabolism and hypertabolisms exist (slow and fast) but these are rare and typically linked to other conditions/issues.
Typically it's because such people either just simply do not eat enough, or fidget and are more active than they realise. Those that have a "slow" metabolism, typically eat more than they realise, and are not as active as they think they are.
I'm afraid that's the plain simple truth. The majority (I don't know the percentage but it's very much the majority i.e. pretty much everyone) have a very typical basal metabloic rate (BMR) which fluctuates a little up and down, but well within normal tollerances, hypo and hypermetabolisms are rare.
That I'm afraid is the fact of the matter!