My speed is related to the proximity of the deadline. I've been known to completely change the plan of a project just before a presentation. Design, and in some ways this is similar to programming, it's a discovery process. By the time you've put an idea into some kind of tangible form you realize a better way to do it.
It's a process, but it's a process that while being informed by your relative experience it's still quite random.
At some point though, the deadline, you have to say it's good enough.
