All of the reincarnations have the n%I=W skill. What does it do? I don't think it was actually explained in the anime, and it was pretty much kept as a mystery.
This was explained by Administrator D in the fourth volume. It seems this skill is what allows them to retain their memories of their past life. It also seems to be why their stat growth rate is so high, however, this may only be an indirect consequence of the skill and not an actual effect (I'll emphasize the point in the passage that makes me think this).
From D's speech on page 103 in Chapter 5: Evolution (emphasis mine):
Left to their own devices, their souls would have disintegrated. That's why I took matters into my own hands and gave them the 'n% I = W' skill. With this skill, they would be able to keep the strength, memories, and such of their souls. Then I gave each of them a free skill based on their aptitude and generously ensured they would be reborn into species with a similar wavelength to their original souls. If you ask me, that was the least I could do.
The reincarnations being able to keep the strength of their souls may be why their stats grow faster than regular people, and thus it isn't actually that this skill directly makes them have better growth rates, though it is an indirect consequence.
Further evidence for this theory on page 144 from Kumoko:
And it would be no exaggeration to say that the power of stats, skills, and so on in this world is actually the power of the soul.
It's possible there may be more to this skill to be revealed later, but this is a pretty good explanation for now, and it isn't even much of a spoiler.