Look, if Kizaru says "have you ever been kicked at the speed of light?", I have no reason to believe he's lying.
lolol. To begin I'm not sure if that translation carries same context as what he said in original japanese language. But that doesn't really matter, because not everything what is said by characters in manga is supposed to be fact. What about Usopp, isn't he lying all the time?
If you ask me then I can guess that it maybe is a catchphrase for Kizaru. But regardless of what he said it doesn't look like he actually kicked at speed of light, or does it look that way for you? I'm confused, because if that's "speed of light" then I'm not sure anymore what speed of light even is for you.