Cornish Rock broiler growth rate

I’ve had the Cornish x for several years now, when I fed them broiler ration they certainly died early, I soon switched to raising them on regular chick feed with the rest of the layer chicks and that early mortality just stopped. Their growth was just a bit slower which allowed their feathers to keep pace with their body growth and they looked...more normal.
My birds were truly gross the first year, half naked, covered in poo because all they wanted to do is lay down in or near the feeder and eat, they hardly moved and were impossible to keep clean, and then they started to die. I didn’t even want to raise them again because I thought it was cruel the way they had to live.
Once I switched feeds and started keeping them the same as the other chicks, they thrived. Totally different birds. They were so active, they free ranged with the other guys and acted like normal chickens, but huge! I was able to keep them a little longer to allow for better weather and time for butchering. The meat was great, carcass size was perfect and I didn’t measure but the carcasses seemed less fatty than before