The speed and snap were gone. Despite winning Kell the fight before it his body gave out, Kell needs to retire.
Golovkin did ruin Brook. I think that's fairly clear. After the Golovkin fight, I think Brook's trainer said the injury Brook suffered was like a car crash injury and Brook could have lost his vision. Brook was brutally demolished. That doesn't go away in 8 months. Physically it might, mentally it does not. Brook is done.