I noticed it calmed my anxiety markedly to breathe that way. That’s short-term, though. The trick is keeping it up.
Re the metabolism slowing down, it makes me think of reaching a state of transcendental consciousness, pure consciousness, through meditation. In that state, breathing slows way down and can even be suspended, and I think other metabolic processes are also slowed or suspended–digestion being one of them. Perhaps slowing breathing way down can decrease anxiety by putting one in some degree of such a state, which then also slows the metabolism.
