It seems like the growth slows a bit when they reaches 4" or 5", IMO. My one of my Green Severums was about 2" when I got him 2 months ago. Now he is 4" and didn't get much bigger. Then again...he doesn't eat like a pig. He only has 2 sticks a meal. Once he is full, he just swim off. My other Green Severum grows so fast within a month. Was only 1.5" when I got him. Now he is 3.5". And he eats a lot. The Gold Severum I talked about, her mouth is crooked to the right, so eating for her is a bit difficult, that's why she grows slower than most fishes.
IMO, it has a lot to do with how your fish eats.