One of the big traffic signals that manages that is these hesitation markers like “um” and “uh,” because they can be used as early as you like. Of course, they don’t have any content, they don’t tell you anything about what I’m about to say, but they do say, “Wait please, because I know time’s ticking and I don’t want to leave silence but I’m not ready to produce what I want to say.”
There’s another important reason for delay, and that is because you are trying to buffer what we call a “dis-preferred response.” A clear example would be: I say “How about we go and grab coffee later?” and you’re not free. If you’re free and you say, “Yeah, sure, sounds good,” that response will tend to come out very fast. But if you say “Ah, actually no, I’m not really free this afternoon, sorry,” that kind of response is definitely going to come out later. It may have nothing to do with a processing problem as such, but it’s putting a buffer there because you’re aware saying “No” is not the thing the questioner was going for. We tend to deliver those dis-preferred responses a bit later. If you say “no” very quickly, that often comes across as blunt or abrupt or rude.