@KG this is just a summary and my last reply on this subject. It was more elaborated first lol.
Kakashi couldn't see the counter because his vision was disorted at high speed. Meaning that, the oponents counter was in fact inside his field of vision, but since it was disorted because of the speed, Kakashi couldn't see it as a counter.
Kakashi's eyes couldn't interpretate what that disorted movement was, ignoring it was a counter from the enemy, but kakashi was in fact seeing it. How come? He could see it (see it and interpretate ir as a counter) when he gained sharingan, which doesn't increase one's field of vision, it just increases reflexes.
So kakashi was seeing the counter, he was making visual contact with it, but his eyes couldn't interpretate it due to the distort at high speed.
What is reacting? Its a chain of actions. Since you see with your eyes, the infro is chanelled trough your brain, and then you get the response.
That's what failed to Kakashi, so, he in fact couldn't react to the enemies counters, because he couldn't see it, but it was in fact on his field of vision.
Gai's speed could generate this tunel vision too, since his naked eye is a naked eye after all, and his speed is greater than Kakashi when he performs chidori, but this is out of subecjt, i'm just saying it is possible that at high speed, you can't react to some speed lower than yours.
Reaction speed has nothing to do with striking speed at all.
Ay's speed > Sasuke's. But Ay atacked him so fast that Sasuke, with a lower speed than Ay's, made a counter and stabbed him in the chest.
Gai not letting Madara make a counter has nothing to do with he being able to react to this counter.