In the script of the episode 7, season 7 there is the following statement:
| “ | But then a stranger sound comes from the north. Tormund looks to the sky, and sees a mote on the horizon, coming in fast. Very fast. It's moving so fast we don't get a clear look at it or the Night King on its back before it is upon us... hitting the Wall with a blast of blue flame. A massive chunk of the Wall breaks free ahead of Tormund. The ice dragon glides away, and we get our first clear look at him, and at the Night King on his back. | „ |
I will calculate the speed of the dragon. For the timeframe I'm going to use the scene from the show, Tormund hears the dragon at 1:31 and it arrives at 1:37, so it would be a timeframe of 6 seconds. For the distance the script states that Tormund sees a mote on the horizon that is coming very fast, so I'm going to use a distance of 10 km (10,000 meters) due to the weather conditions at the scene. Putting all the data together, the dragon would have a speed of:
10000/6 = 1666.667 m/s or 6000 km/h (Supersonic+)