Some possible ways to respond to a student complaining about not having enough time on exam

A student emailed me after a recent exam, complaining about not having enough time to finish the exam. Out of all the students, most managed to finish within 2/3 of the provided time. I wonder what might be some possible ways to respond to this kind of complaint. (This is my first time teaching a class of my own -- I only TA'd before, so I've never had to deal with this type of issue in the past).