The Designer's Law of Diminishing Quality
Okay, so there's no official "law" called "The Designer's Law of Diminishing Quality," but it very much exists in the design world.
Here's the gist: the tighter the deadline, the lower the quality. Meaning, for every day that you want your project finished faster, the quality and attention to detail drops accordingly.
You want that logo designed in a week? Fine, you can have something generic that "looks cool" in a week. But something that's well researched and thought out just won't happen in that amount of time.
You want a full site redesign done in two weeks? Okay. Again, you'll get something generic that probably will satisfy you. But it the little things that make a design great and more importantly, usable, will be left out and you'll most likely feel the pain of that decision with a higher support load or lost users.
Good design takes time, and not just pure grunt work. Letting a design sit for a couple of days and coming back with fresh eyes does amazing things. "But letting a design sit means I get my design later," you say. Yes, yes it does. But it also means you get a better end product.
It's worth noting here that there does come a point in the life a design where you've just got to call it done and move on. Trying 37 different shades of fuschia is, in most cases, a waste of time. And most designers would tell you there's always more they'd like to do on a design. From a client perspective, you've got to trust the designer to know when a design is finished and give them time to reach that point. You'll both be happier because of it.