It's the same as any other higher-level skill: the faster you can perform the lower-level functions, the faster you can iterate.
Your medium of expression also influences the speed of your thought. You'll notice that people who speak for a living seem to be wittier than those who don't; they're able to form thoughts and translate them into expression very quickly. Typing/writing is already much slower than speaking, but its advantage is the ability to revise a thought before conveying it. If you're typing slowly, you're probably forming thoughts slower than someone who types quickly. Anecdotally, spending so much of my time communicating via keyboard stunted the development of my ability to express vocally, because it lowered the rate at which I had to generate and, internally revise, and expel thoughts. It would be even worse if I could not touch type.
Your medium of expression also influences the speed of your thought. You'll notice that people who speak for a living seem to be wittier than those who don't; they're able to form thoughts and translate them into expression very quickly. Typing/writing is already much slower than speaking, but its advantage is the ability to revise a thought before conveying it. If you're typing slowly, you're probably forming thoughts slower than someone who types quickly. Anecdotally, spending so much of my time communicating via keyboard stunted the development of my ability to express vocally, because it lowered the rate at which I had to generate and, internally revise, and expel thoughts. It would be even worse if I could not touch type.
That's my layman's take, anyway.