Mine grow slow as well, at least the ones I believe must be females are light eaters/slow growers. Of my thirty gibbs, I've had twenty-four for 1 year and the other six have been in the tank for 4 months. All were 3"-5" when they arrived.
Males growing from 5" to 10-11" in a year is actually pretty impressive I think mate. Having good protein rich foods, lots of room to move around & great water parameters are prob why you have seen such good growth on the males especially. I would be totally stoked to achieve anywhere near those outcomes!
A friend of mine has been running chemipure blue in his 220gal w a group of 9 wild & 6F1 Kigoma fronts and his nitrates have been almost undetectable, even letting his wc schedule go out to 2 weeks. He still does his big water changes at the 2 week mark to remove other metabolites & reduce the bacterial load from the water column. Depending on cost & life-span I may introduce it to my system, especially since I have 10ppm nitrate straight out of the faucet.