F-air and I think U-air are all faster than N-air.
try doing a SH insta-F-air and starting another aerial. N-air won't start before you land, but F-air and U-air will (I think).
N-air is kind of... not disjointed, so good luck on breaking through aerials with it, since you have to 'hit their hitbox first'. The best way I can think of to set it up so you're next to your opponent's hitbox is to do it out of a spindash combo. It's far more reliable than F-air for damage then because people won't DI out of an N-air since it's only 1 hit.
You misinterpretted my post ='( =P. I didn't imply that Nair could "break" through other aerials. I actually kinda implied the opposite. I made a misteak in my original post though, you have to hit your oppnents hurtbox, not hitbox first. But anyways what I said was "use Nair if you're in right in someones face, particularly if that char has fast aerials". I was saying how it could be useful if your opponent is right next to you. In situations where you are relatively close to the ground, I would consider it a better option than fair or uair, because you can fastfall it without horrendous landing lag.
As for the attack speed however, I'm not sure of totally sure either, but f anything its not considerably slower than fair or uair. I just say this because I do a lot of random testing in the WiFi practice stage, and if I was close enough to sandbag, it seemed to come out immediately as I pressed A, but that isn't exactly quantitative data lol.
Anyways, I still think Nair is a good option if you are in your face. It isn't going to win battles in the air, but is useful from shorthops, or from the ground imediately into an aerial. I find its knockback in general to be good to string into(in any of the three hits), and once more, negligible landing lag.
EDIT: I know that DJbrowny's method of frame data has been questioned, but it should still function well in terms of relativity within Sonic's own moveset. According to his data, Fair comes out at frame 4, while Uair and Nair come out at frame 5, so I guess you were kinda right. He says Nair has 13 frames of landing lag, which I think is too much, but not sure. So yea, Nair is still useful because of its low landing lag, longlasting hitbox, and stringable nockbacks imo. But in the end it comes down to player choice.