> Well, if you're the kind of person who thinks a lot in private -- then, in conversation, you can respond quickly and deeply: because your thinking has already been done.

I also disagree with this, and I'll provide slightly different reasoning, although I think it does overlap with what you're saying regarding evident utilisation of the mind.

Even if someone has done a lot of thinking about an issue, they may want to take time before responding in a conversation to make sure that they've correctly pattern-matched the circumstances, and to prepare a suitable response for the moment and audience.