At about warp 6.3 you're going twice as fast as a warp 5 ship. Doable, but you're also tapping into your more highly skilled spacers to crew this as a fast cargo ship. Getting additional dedicated cargo ships with a sensible, manageable warp 5 cruise out becomes even more necessary.
Also, from what I can tell, Cardassians prefer to have a steady supply of a given ship class. With n-Jalduns , m-Kaldars, and k-Takkais a year, that sort of thing, as opposed to Starfleet's habit of throwing out waves of our newest toy. It's an interesting show of differences in handling the problem of ship building scheduling.