In this scene, after Kai throws his blade, he retrieves it from a very long distance. I will calculate the speed of this.
First I need to know Oogway's height. In a previous calc, a user found Kai is 3.3579 m tall, based on Po being 1.8796 m, so I already know his. Oogway is roughly 1.20 m tall based on this image.
_English_0-23_screenshot.png "Kung Fu Panda 3 - Kai vs Oogway HD (Blu-ray 1080p) English 0-23 screenshot.png (1.15 MB)")
_English_0-22_screenshot.png "Kung Fu Panda 3 - Kai vs Oogway HD (Blu-ray 1080p) English 0-22 screenshot.png (1.86 MB)")
With the first image, I found out that the structure Kai is standing on is 24.6 m wide. With the second, from afar it's 28 px wide and Oogway is 576 px tall.
Formula: 2atan(tan(70/2 deg)*([object size in pixels]/[screen height in pixels])) = 2atan(tan(70/2 deg)*([28]/[812])) = 0.0482807942 rad or 2.76628574 degrees
Distance = 509.42 m
Kai recalled the blade in a single second, meaning he very casually moved it at 509.42 m/s. Granted, it's slower than the cannonballs Po caught in KFP2 (which were moving at 758.95200 m/s) but it's still Supersonic and done effortlessly. If we assume he moves twice as fast when enraged or actually trying, that would make him Supersonic+. Just a 'possibly' though, since there's no way to know for sure how much faster he is in battle.