I think the compassionate and respectful thing to do is the let them finish. I would think if someone senses your frustration, they could become nervous about your annoyance and it could make their stuttering worse. Do you really think you’ll be late some where or is it more of an anoyance? I dont want this to sound mean but I’d say weigh your lateness against their self-esteem. Unless you would finish someone’s thought, no matter their speach patterns, because you’re late it would show a lot of character strength to let them finish. This could be an opportunity for you to be very loving and compassionate, TAKE IT!!