"He has to channel his magic at a very slow pace while accessing the ward, because it absorbs magic like a black hole. The Dementor is a siphoning device It makes so much sense now.  He gives his core time to replenish itself as he spends his magic. Once he is inside he will be one with the ward. He will have access to what caused the security systems failure so he can repair it. That is another painfully slow process." 