Peter felt as though a cat was about to drive him mad. He hadn't exerted himself this intensely in a very long time.
A rat had limited explosive speed. Even if that rat was originally a wizard, once transformed into a rodent, its physical stamina and bodily capabilities were practically indistinguishable from a real mouse.
Peter knew he had to shake off the crazed cat hot on his tail as quickly as possible, shake off Ron who was still chasing from behind, and above all, shake off Sirius Black, who could appear at any moment. He needed to escape the grounds of Hogwarts and find somewhere to Apparition away.
Chapter 726