I have RIR,barred roc and white I'm not sure what kind But the white ones are growing 2 times as fast as the other two are.
Could be...... just could be that the "white" birds that are growing so fast are meat birds, ie CornishRockX. Pictures would show us for sure.
You might have Light Brahmas. My light Brahmas out grew the EE, RIR,BA chicks of the same age, and at 2 months they are as big, maybe bigger, than my grown Brown Leghorn hens. Just a guess, but it might be meat birds too.
Maybe a white Leghorn? I have some Leghorns and some Plymouth Rocks, and the Leghorns are definitely growing fastest.
It would be good to post pictures, and where you got the birds (hatchery, swap meet, farm store, ect.) Unfortunately, people accidentally get meat birds mixed in with their regular chicks unknowingly, and they are not meant to live past a few months without becoming crippled or dying of heart problems. It happens all the time to unsuspecting people.