No. They in no way slow metabolism. The effect on the CNS is not the slowing you're thinking of, the body requires a specific amount of calories each day (google TDEE calculators to find your actual metabolic rate). If you take benzos, they will not change the amount of calories your body requires. That is determined by your age, weight, muscle mass, and activity level. No drug changes the metabolism not one calorie, if that were true, there would be actual fat burners in existence that actually burned fat lol.
Indirectly, you may find yourself not as hungry while on benzos, though. But no amount will change your metabolism.