Do not daven slower or faster than what is acceptable. Do not place "a burden on the congregation." If you daven too slowly you will cause others who have to go to work to leave before the end of the davening, and you will prevent the next minyan from starting on time. If the someone davens too quickly, do not admonish him in the middle of the davening and embarrass him. Talk to him as a friend after davening. If speaking to him gently does not work, do not ask him to lead the davening.