Let's do some math here.
Page 7: Kanan grabs Miltie and turns her around after 0.0024 s of distraction
Page 8: At 0.0256 s of distraction, Kanan has spanked her sister at least four times.
(0.0256 - 0.0024)/4 comes down to her taking at most 0.0058 s to smack that tush once
I cannot say for certain how much Kanan moved her arm here, but even if we guess and say the distance from Kanan's outstretched hand to Miltie's bum is 0.5 m, then the basic movement of her coming in for the slap and then coming out would clock in at about 1m/0.0058s = 172 m/s
Which means that Kanan is smiting her little sister's dump truck with a hand moving about half the speed of sound
EDIT: I did a whoopsie-doodles and misread the panel - Kanan spanks at least SEVEN TIMES
Which means that a single smack takes 0.0033 s and therefore comes at approx. 303 m/s
Which is close to MACH 1