thanks,
Base on my experience, I think that all sens do grow slow, they grow fast till they reach around 6" then they slows down. They get fat faster then their growth.
I have a couple senegals which I bought when they were around 4", I've kept them in the same tank, fed them the same food, but still noticing a very little growth on them, perhaps considering their maximum size which only could get up to around 11"-12" in a home tank.
cheers