I think we have to be a bit more specific when it comes to slow or fast growth... if we don't compare it to something then the adverbs slow of fast are vague and is biased by the person that's reading it, plus all tanks are different and even two frags from the same colony, if put in different tanks, will probably not grow at equal rates! Although I don't have much experience with Zoa's but I do have a PPE colony that's been with me for around 4 years. It had from what I can remember it had around 5-10 polyps when I accquired it and since then has grown into a +or- 60 polyp colony... so subtracting the initial polyps and dividing it by the time i've had it.... I estimate a growth rate of about 12.5 polyps per year or 1 per month!... it's up to you to decide if that's fast or slow.
here is a pic... enjoy