Seiya’s Pegasus Meteor Fist hit Dragon Shiyru when he was posing for his ultimate attack
The timeframe is one millisecond, as it was stated that is how long there is an opening for his weakness to be apparent
Now I need to calc the distance
Seiya is 165 centimeters tall, and his height here is 204 pixels
Panel height = 1200 pixels
2*atan(204/(1200/tan(70/2))) = 9.21029384 degrees
Seiya is 165 centimeters tall, and the angle is 9.21029384 degrees
http://www.1728.org/angsize.htm
That equals a distance of 33.6023622 feet
Now Dragon Shiryu is 172 centimeters, that is -5.64304 feet
or 27.9593222 feet
27.9593222 feet/1 millisecond = 19,063.1742 miles per hour, or mach 25.0433495 (Barely high hypersonic)
I am not sure if this is consistent or not yet, as I am only on chapter 11. But this calc would also count for Seiya's reaction, also it took him a millisecond for him to realize and conceive the weak-spot, charge up his attack, and for the attack to get to Shiyru. So this calc is a lowball assuming it took a full millisecond for the attack to reach Shiryu.