Seems 5.3 introduced a new bug with pets, I found this out when I was soloing the new instances pets would stay behind and despawn. The problem is this pets stay behind after combat crawling at slow motion and in-combat they seem slower than normal aswell. Tested with service and supremacy pets.
It's the same for hunters and it is not a new thing. It happens when you exit combat and your pet is far away from you. After it crawls back to your side it will move at full speed again.