No they are not slow growing. To get the maximum growth they love daily waterchanges. They feed heavily and prefer to eat a LOT and OFTEN. Of course that fouls the tank, so daily waterchanges of at least 20% will help.
Mine get daily waterchanges of 20%, 2x weekly they get 50%. There were 5 in a 55g, now there are 3 after the heatwave and my laziness due to the same heatwave, and a discus with a sensitive tummy.
Go easy on the flakes, concentrate on frozen or live foods. I"m breeding convicts to supply my discus with live food.