I've never personally seen anything on the effects of a Benzodiazapene on the metabolism's functioning slower or faster but, I do know that when I was on xanax or more often the Klonopin, I just didn't have the energy to do anything. This is probably less to do with the metabolism than it's sedative effects though. I wouldn't worry about it. I would look into the side effects of the SSRI's more. They have more studies showing
weight gain effects, sadly.

I just work out harder to counter it really.