If we are speaking off the cuff as opposed to giving a rehearsed speech...it is natural to have some “ums” and “ahs”, as we are thinking as we speak, as this article says. If it’s a conversation or debate we are engaged in, and not a monologue, then I should think it would be impossible not to give ourselves thinking or response time by umming and ahhing. I think professional speakers, which is really what this article refers to, are in a different category altogether because they are paid to be fluent and amusing and are usually engaged as after dinner speakers purely on that basis.