I was walking over to Building 40 to get lunch this afternoon, and my iPhone died, so I was forced to listen only to myself think for a few minutes. My life has become so media-saturated that that's actually a fairly unusual circumstance, and after a bit, I decided to see what would happen if I just started talking out loud.
One thing that I noticed was that I had a hard time exactly translating the thoughts in my head into words. I'd get the gist out, but in my head, perfectly-formed phrases and subtle insights bounce just out of my grasp, uncatchable by my voice.
I realized that this might be due, in part, to the fact that I've done a lot of self-censoring over the years; that not everything I think ever gets said (and probably most of it doesn't). Over time, that mechanism probably became somewhat automatic and so now, even when I want to turn it off, it's not quite so easy.
In addition, there's probably inevitably some lossiness in the translation between media - thought to speech and thought to text - just like how some English phrases don't have French equivalents, and vice-versa.
Which all means that most people are probably more eloquent in their heads than they are when they speak, and one shouldn't be quick to assume something about a person's relative level of intelligence, based solely on their ability to efficiently funnel their thoughts to their throat.