Hold on there. Let's get this on the record correctly. Cannabis neither speeds nor slows the metabolic rate. Metabolic rate has nothing to do with what people smoke. It has to do with lots of other factors like muscle mass, caloric intake and expenditure, percentage of body fat, thyroid function, age, etc. Cannabis may affect caloric intake and appetite, and it certainly can affect cardiovascular function, but there's no evidence it has a direct effect one way or another on the basal or resting metabolic rates.

You can read some basic information about metabolic rate, which is what people mean when they say "metabolism," here:
Basal metabolic rate - Wikipedia, the free encyclopedia