In this scene, the dragons fly from the ground to the top of a snowstorm. The type of clouds that produce snow are Nimbostratus, which usually are 2000 to 4000 meters high, with 3000 meters being the average. A nimbostratus cloud has an altitude of between 500 to 5500 meters (average of 3000). So, in total the dragons flew 6000 meters from the ground to the top of the snowstorm. The dragons started flying at about 0:17 and arrived at 0:34, so that's a 17-second timeframe. So the dragons would have a speed of:
6000/17 = 352.9411764706 m/s (Transonic)
Kinetic Energy[]
Viserion and Rhaegal have a weight of 1,909,835.557 kg:
0.5*1909835.557*(352.9411764706)^2 = 118,951,695,591.7034318431 Joules or 28.430137569718791468 Tons of TNT (8-B: City Block level)
Drogon has a weight of 3,300,195.8425 kg:
0.5*3300195.8425*(352.9411764706)^2 = 205,548,529,982.712665173 Joules or 49.127277720533619743 Tons of TNT (8-B: City Block level)