Toz and the others were getting close to their destination, the Fortress of Death. And the increasing density of death attribute demonic energy was proof of that. Usually, they would have wanted to get through it as quickly as possible. But Hals´'s humming calmed them down, and they realized that they had to take it slow to reduce the risk of discovery.
If they were found before they could even enter the fortress, the plan would be bust, and they would have failed their mission. Even if they wanted to get it done quickly, rushing would lead to nothing but their deaths and the mission's subsequent failure.
