Note: Credit where credit is due, the original calc is by Mephistus. I moved it here for the sake of having it on-site.
Sonic (a classic version of himself technically(?!)) does the spin dash attack to gotta go fast to move fast whenever he wants pretty much, unlike S&K3 we can compare the charged up version and non charged up version instead of just a baseline spin dash; we can compare his movement speed here on screen directly to the velocity of the Eggrobo's lasers they shoot at him, which act as natural light would be expected to; lasers travel at the speed of light by name description of being light.
"These robots are one of Dr. Eggman's creations from the Sonic the Hedgehog series, where they tend to hover in the air and shoot at Sonic with their laser guns. In this game, they'll deliberately keep their distance while firing both short and long beams at the fighters. Get below them to avoid their fire, and then strike!"
Non charged spin dash:
https://youtu.be/ulHCgHZ3nsI?t=274
50 - 36 = 14
Laser beam moves 14 pixels in 1 frame
381 - 302 = 79
Sonic moves 79 pixels in 1 frame
The speed of light is 299,792,458 meters / second
Sonic's movement speed with the spin dash = 299,792,458 * (79/14) = 1,691,686,013 m/s or 5.64285714286 times faster than the speed of light
Charged up spin dash:
https://youtu.be/ulHCgHZ3nsI?t=403
174 - 65 = 109
Using the same images as prior, the light beam moves 14 pixels in 1 frame
Sonic's movement speed with the spin dash = 299,792,458 * (109/14) = 2,334,098,423 m/s or 7.78571428571 times faster than the speed of light