[fasterik]

I think speaking fluidly while thinking out loud is a completely separate skill. Some people are really good at it, usually the ones who get a lot of practice at public speaking. I also suspect extroverts have an easier time with it than introverts. "Ums" and "ahs" aren't necessarily evidence that a person is thinking, but it's also true that a lot of very smart people are "inarticulate" in the conventional sense.