Certain breeds of meat chickens only take about 12 weeks to reach harvest age/weight.
Cornish Cross, the commercial meat breed, takes 6 weeks to processing time.
A friend of my son’s moved away and gave son a pair of chickens rescued from the meat farm. These things were morbidly obese and could not even climb up the ramp into the coop. One just dropped dead one day. The other became so pathetic and torn up by the rooster that both were dispatched.