Excerpt from Paul Graham:
“Being a really good speaker is not merely orthogonal to having good ideas, but in many ways pushes you in the opposite direction. For example, when I give a talk I usually write it out beforehand. I know that’s a mistake; I know delivering a prewritten talk makes it harder to engage with an audience. The way to get the attention of an audience is to give them your full attention, and when you’re delivering a prewritten talk your attention is always divided between the audience and the talk—even if you’ve memorized it. If you want to engage an audience it’s better to start with no more than an outline of what you want to say and ad lib the individual sentences. But if you do that you could spend no more time thinking about each sentence than it takes to say it.  Occasionally the stimulation of talking to a live audience makes you think of new things, but in general this is not going to generate ideas as well as writing does, where you can spend as long on each sentence as you want.”
See the full post: Writing and Speaking.