Aircraft are limited to a speed of 250kts below 10,000ft unless they're in an airport's control zone and then they're limited to 200kts. So, given the time to navigate Frankfurt's complex airspace, as well as balancing how much gas needs to be burned to get high to go fast, when you will shortly need to come off the throttle and descend anyways, versus just staying low and within the speed limits, I can see 55 minutes being well within the realm of reason.
The shortest commercial flight I ever took was from HOU to IAH†, a flight distance of 24mi/39km. I was surprised how long it took. At first I thought it was because of traffic, but a member of the flight crew told me, "We go low and slow."
†This was back when Continental was all mad about the parking situation at IAH, and so if you had an outbound flight from IAH, it would let you park for free at HOU and then add on a free HOU→IAH segment. At the time, you used to get 500 frequent flyer miles just for getting into the air, so it was an option I exercised as often as I could.
I can tell you that the seats were all on one side of the plane. It was a proper Continental craft. Not a puddle jumper like I've been on going to remote places in the northwest.
