Berachos 34a

Gemara: "Once a student went down to pray for the congregation in front of Rabbi Eliezer and he was being overly lengthy. [Rabbi Eliezer's] students said to him, 'This person is a real lengthener' (couldn't think of a word - sorry). He said to them, 'Is he taking longer than Moshe Rabbeinu...' "
A source for this concept that one who is the Shaliach Tzibbur should not be overly lengthy (Rabbi Eliezer's students' contention), is from a gemara earlier, on 31a: "Rabbi Yehuda said: 'This was Rabbi Akiva's custom: When he would pray in the synagogue he would shorten [his prayers] and go up [from the Amud] because of inconvenience to the public...' " Interestingly, Rabbi Akiva himself was a student of Rabbi Eliezer.