"If you can't pull the process together to make a quick decision, you're not only going to lose people, but you're going to signal that you can't move quickly on other decisions as well, whether or not this is really the case."
This one is important. Recruitment leaves impressions on the candidates. You better make sure the impressions are positive ones, because they matter a lot.
The world is small. An unsuccessful candidate might get successful elsewhere. Their opinion about your company might soon affect its future. Other candidates passing on your offer because they heard your process sucks is just the most obvious and immediate way, obviously there are others.