I know this is a bull attempt, but this is a bit wrong here m8. You forgot that brain needs 100 milliseconds to process the image. The photons, e.g. light uses cornea and lens (mostly the cornea) to bend the light onto the retina. The retina takes the light and changes that stimuli into electric info that the optic nerves send to our brain. Vision "happens" in our occipital lobe, which is at the back of our heads above our neck. 100 milliseconds from 4 meters is pretty much a joke for Gai's speed.
Humans cannot see light speed (which should be obvious). Gai needs to process the image before such is done, and he'd be more than done before he was to be trapped in genjutsu.
I'm not sure where you're sourcing your information but been recorded that the Human brain can process an image in as little as 13 milliseconds. This is still under the time it would take Gai's nerve impulses for muscle position to complete their journey as well as for him to travel to Itachi.