If ur looking for a faster growing animal, you need an early-maturing breed. In the feedlot, I generally find the british and australian breeds growing the quickest. I'd really suggest angus, whiteface, shorthorn or murray grey, although shorthorns steers tend to need a smaller space to finish and by whiteface I mean horned or polled hereford. These breeds tend to be medium to small frames, although you can easily get the shorties and blacks up to 600-700 kg liveweight.
Later maturing animals are slower growing and are usually bigger. Things like your big, ultra-muscled euro breeds eg. Limos, charolais etc. You know the saying "it wont happen overnite but it will happen" well that applies to these breeds. Although alot of these animals are HUGE with GREAT muscling (usually A+ or A muscle scores) I basically dont use them at all for feedlotting, except in extended feeding programs for the heavy export market. However, they are useless for the vealer/yearling domestic market.
And don't even consider bos indicus cattle.
:cboy: One interesting thing to think about is the new breed Square Meaters. I find that out of all the 'miniature' breeds this one can actually stand it's own against 'regular sized' cattle. And they grow so fast it's like it's happened overnite.