Apropriate lighting (some zoas like tons, other like the shade), good indirect flow, stable water parameters, food source (and not necesarily anything you have to add to the tank yourself), and the proper placment in your tank all= great growth.
Pretty much all comes down to good tank husbandry, proper equipment and proper placment in you tank, and they have the potential to grow like weeds- even the said "slow growers"
You find a zoas happy spot, and they will explode for you, in your tank.
Sound easier than it is, unless you are really lucky!