Perhaps you're thinking of the difference between whiffing an attack versus hitting with it (shield or person). If you use an attack and it doesn't hit anything, you'll land faster. If you use an attack and hit someone or their shield, you are stuck motionless for a couple frames more.
(smakis beat me to the above)
Here is a breakdown of how fast Ness's DJC is:
DJC (double jump cancel) with no hitbox and no landing lag
1. Jump on frame 1
2. Jump on frame 4
3. Do any aerial on frame 5
4. Frame 6, you can do anything
Basically, you'll just make a little white circle come out and the double jump sound, but you won't hit anything.
Special DJC (no landing lag)
1. Jump on frame 1
2. Jump on frame 5
3. Nair or Dair on frame 6
4. Z-cancel if you Nair anywhere between frame 7 and 11 (You do not have to Z-cancel a Dair)*
5. Frame 12, you can do anything
OR
5. If you didn't Z-cancel the Nair, you are stuck in Aerial Landing lag (move on frame 26)
*Note: If you do a Bair, Fair, or Uair, you will not get any hitbox, but you will not have landing lag either
Normal DJC Nair/Dair (must Z-cancel)
1. Jump on frame 1
2. Jump on frame 5 or 6
3. Nair or Dair on frame 7
4. Z-cancel on frame 8 through 16
5. Z-cancel landing lag for 3 frames**
6. Move on frame 20
**If you didn't Z-cancel, it's probably something like frame 30 when you can move.
Normal DJC Fair/Bair/Uair (must Z-cancel)
1. Jump on frame 1
2. Jump on frame 5, 6, or 7
3. Fair/Bair/Uair on frame 8
4. Z-cancel on frame 9 through 18
5. Z-cancel landing lag for 3 frames
6. Move on frame 22
These are the fastest aerials you can do. Basically...you'll probably have the best luck breaking shields with down air or neutral air. I'd suggest Dair because you can keep the control stick down the whole time.