Nothing grows as fast or converts feed as well as a CornishX.
I doubt anything other than a hybrid will come close to the growth rate of a cornishXrock if that is your yardstick.
In my opinion, freedom rangers, which I've raised a couple times and currently have a flock grow quickly with 22-24% protein and still forage well.
Anything else will probably grow too slowly for you. But the flavor, due to being older will be better.
New Hampshires, Delawares and Rocks are probably your best bet for a fast growing DP bird.
Buckeyes and Javas are quite meaty but slower.
Great foraging DP birds that also are known for flavor are Andalusians, Black Penedesencas, Sussex, RIR, Wellsummer, Orpington, Manx Rumpy, La Fleche, JG, Holland, Dorking, Dominique and Faverolle. The issue with these is you won't get the growth rate or perhaps carcass shape you're looking for.
I've done half of those and had associates do the others.
Another thing - slower growing/heritage/free range birds CANNOT be cooked the same. They must be cooked low and slow or will be tougher than the melt in your mouth meat of the Cornish Rock. The meat requires chewing.
Edited for spelling