Kizaru’s light already diverges from irl light in many other ways, so why should we assume it’s moving as fast as irl light?
Take in it takes Loki to use his lightning and Ragnir to use his ice combined to create light/lasers.
Just like it took Big Mom to combine fire, lightning and her blade to mimic lasers.
Anytime elements are on screen the Kizaru upscale goes crazy.
There are two elements that are used to show a superior element in the verse. Light and Magma. Oda uses Magma to show a higher level of flame similar to how he combines elements to create light.
Ice and lightning are a tier below them as shown by Loki and Ragnir. However the thing with Kuzan is he's not an ice man, he's a cold man, his actual ability is just lowering temperature, so while his produced element is clearly weaker he can probably just lower the temperature even further to cancel out stuff like magma (idk about lasers he's kinda cooked on that front).