Just a thought. I can speak dozen of words a minute in more or less complete sentences. Even very fast writing is comparatively halting, hesitant, disfluent. Not because of the effort of writing or typing it, but because writing is something fundamentally different. Fast writers like Kerouac are actually better than the normal writer who can barely write anything with any coherence.
The other side of that is that we hold writing to a much higher standard, forgiving the disfluency of spontaneous speech. If we wrote as we spoke, then we would say, hey, why didn't he revise that? It looks like he just wrote down what he was thinking with no care put into the process.