[@Klomster] Humans have good reaction time to begin with. Once something, say, touches us, it takes fractions of a millisecond for the nerve endings in the body to send that information to the brain. From there, the average person can react on instinct within the next second, depending on the input. Say someone grabbed a hot pan from an oven. The average person could react quickly enough to let go of that pan before it caused anything more than probably some blistering. Given it's a race against energy transfer, that's pretty damn quick.

However, literally anything that has a nervous system can do that. Yes, the amount of time it takes the average person to act on information they perceive through their senses is a very small amount of time... But so can anything. All being faster than a human at reaction time means is you're shaving off, in the most extreme cases, a second to two.

A space marine can draw his weapon faster than basically any human alive. However, given that things like lasers exist in the 41st millennium, that doesn't mean much. Yes, space marines react faster than humans. Nine times in ten, though, and especially in a sci-fi setting, the dangers they're reacting to are so fast to begin with, that the damage has already been done before the sensory organs they rely on even have time to translate the incoming data into a form the brain can read it in. In other words, they literally wouldn't know about it until it has already hit them.

However, in 40k, there are things that can react even faster. Eldar can dodge lasgun fire. Necrons can phase to another dimension as they're being shot. Daemons can deflect bullets. How? Space magic!... But, in a world where that's a thing, space marines are far from the end-all be-all that the books like to make them out to be.