It took a full 10 turns to build something that my regular fleets could do in one or two turns. It never made sense to build these things.
Build them next to the star, and use them as your initial declaration of war, protect them with Dreadnoughts until they're ready, problem solved.
They had several advantages over normal planetary conquest. The planets destroyed couldn't be reconquered, it created more asteroids to mine, and you could use range limitations against your enemy by creating buffer zones. Not to mention that by using them as your method of declaring war you could wipe out large chunks of them quickly.