On the battle forums, there has been a lot of debate over whether or not Naruto is lightspeed (in terms of reaction/combat speed.) The idea that Naruto may be lightspeed comes from his dodging of a technique called Sage Art: Light Fang, which was used by Madara (after Madara was amped by the Sage of 6 Paths.)
Before you immediately dismiss it by saying "ninja techniques are not as fast as the real thing," which is true, this is no ordinary technique. This technique is, as the name states, a Sage Art, meaning it uses natural energy/energies, not just normal chakra. This isn't the first time we've seen something like this; Sasuke's once increased the speed of a lightning-type attack to that of natural lightning by using the latter - a natural form of energy - to supplement his chakra.
Sages, people who use senjutsu/Sage Arts, do something similar but at a much higher and refined level. Madara, at this point in time, was one of the most powerful Sages, and his chakra was essentially just natural energy. It should also be noted that techniques classified as "Storm Release", are not lightning based as you'd expect; they are all light based attacks.
Anyways, the 4th Databook (which is the most recent and updated of the databooks) confirms it lightspeed:
Ninjutsu / Senjutsu - Sage Art: Storm Release Light Fang
A ninjutsu used by those who gained the six paths power, they emit a to bisect the enemy. Like a sharp and pointed fang, it cuts all things with , making it impossible for the enemy to evade even one swing
So we know the beam is light speed because:
- Official sources told us so, twice.
- It's in the attack's name
- It uses natural energy
That leads to people questioning whether or not Naruto actually dodged the beam, or if he did, that he must've just moved faster than the speed at which the beam was swung, not the light itself. This is understandable, seeing as the original manga scans were quite vague.
However, the episode 424 of the anime proved those arguments wrong. Let's look at the episode's screen caps, shall we?
So, I guess we can now say that Naruto has lightspeed reactions in his newest form. This makes sense considering at this point, he's far faster than the likes of Gai, who has attacked so fast he warped space (which only happens at relativistic speeds, or 10% lightspeed+.)