I'm glad you posted this question. I was looking at them just last week and I wondered about it. I was thinking about purchasing them. Hope someone will come along with info.
From reading the website, it looks like both of them grow slower because they are more active than the Cornish.
Black Broiler
St Run
Black Broilers are efficient producers of broiler meat for those who do not wish to have white plumage broilers. When broiler rations are fed, producers can expect to have six pound broilers in seven weeks or less. Since black broilers are more active than Cornish Rocks and grow slightly slower, they rarely experience leg weakness or Ascite, which is fluid in the body cavity. Black Broiler hens are not a good choice for efficient production of brown eggs.
Red Broiler
St Run
Red Broilers are efficient producers of broiler meat for those who do not wish to have white plumage broilers. When broiler rations are fed, producers can expect to have six pound broilers in seven weeks or less. Since red broilers are more active than Cornish Rocks and grow slightly slower, they rarely experience leg weakness or Ascite, which is fluid in the body cavity. Red Broiler hens are not a good choice for efficient production of brown eggs.
I got 24 red broilers from Ideal Poultry in April. They didn't grow nearly as fast as advertised, though. At 11 weeks, they were just getting into the 5 lb. range live weight. Dressed out, they weighed about 3 lbs. I was under a time crunch (I had less than 12 weeks to raise them to butchering size), so when I bought them I was expecting them to be around 5 lb. dressed weight by 10 weeks. I was disappointed by their small size. Next time I'm in a time crunch for raising meat chickens, I'll stick with the Cornish X's. The red broilers are definitely not "coop potatoes" like the Cornish X's, they were very active like regular chickens.