The figure I heard was that it moves about 15 mph - more than 5 but still not that fast. But remember, we’re talking about momentum here. The ferry is as tall as a three-story building and as large as a football field. The thing weighs over 2,000 tons and when that much mass hits at any speed, there’s going to be damage.
NYTimes has a map graphic of the area. It shows that it hit a pier in a completely different area then where it normally docks. Which seems fairly important to me: it wasn’t just coming in to the normal docking too fast, it was out of control.
According to the NY Post, the boat was going significantly faster than it normally does. That seems to be both the cause of the accident and the reason for the number and severity of casualties.
The ferry was going much faster than it was supposed to, probably close to 20mph, before it crashed. Remember that these boats are very big and carry a significant amount of momentum.
The windows that got ripped were all on the lower deck, which collided with the maintenence peer, which is a little south of the peer it’s supposed to dock at.
I’m guessing lots of people got their limbs severed by falling debris. Many were sitting in seats right next to the windows that were destroyed; they probably got mangled by the peer itself.