The facts are:

The Ironfleet is nearly 40 longships (as far as I remember), which each carry 1000 men, big. Their banners have hundreds of ships but only small longships, who are used for raiding. In fact the greyjoy fleet is nearly as strong as all fleets of the other Houses together, but that's just as it is. THe greyjoys lack catapults and rams for their ships though.

Building ships should not be too fast.