Well, fastfalling increases fall speed by 60% (for Corrin, it's 1.65 --> 2.64), so I think it should be FF FH: 22 frames and FF SH: ~15 frames.
I've been looking at this for a while trying to work out why it
feels incorrect...
I think the error here is assuming that fallspeed applies to a reduction in the total frames including the rising portion of the airtime. I see a few ways to calculate this, but the core of it is that fastfall shouldn't be applied to the rising part.
Shorthop(half) is 18.5
60% of 18.5 is 11.1
18.5(rising) + 11.1 (fastfalling) = 29.6
That comes out to:
SH - 37
FH - 55
SHFF - 29.6 frames (I assume this rounds to 30frames)
FHFF - 44 frames
I'm fairly confident that these are correct because they correlate with some interesting autocancel numbers Corrin has. To jog people's memories and make it easier for reference here are her aerial autocancels:
Nair autocancel - Frame 50>
Fair Autocancel - Frame 30>
Bair autocancel - Frame 42>
Uair autocancel - Frame 40>
Dair autocancel - Frame 41> (not useful)
It's probably also useful noting FAF numbers alongside as they allow you to throw another move before the autocancel, this allows using the startup autocancel numbers (best being fair frames 1-6).
Nair FaF - Frame 47
Fair FAF - Frame 36
Bair FAF - Frame 42
Uair FAF - Frame 38
Dair FAF - Frame 52
Might as well throw our other aerial here too:
Dragon Lunge FAF - 42
I think the interesting numbers here to take into mind are Dragon Lunge FAF compared to FHFF, Bair and Uair autocancels also compared to FHFF, and that all aerials will give Corrin landing lag if performed with a SHFF except fair. This suggests to me that a Corrin SH spacing in the neutral should probably be using fair and not nair when Corrins do not know if they might whiff.
I'd be interested in other takes on looking at things in this manner. I know 13frames of landing lag isn't a lot, but it's telegraphed and opponents can time around the telegraphing pretty well. I've seen Falcon and Sonic notably punishing Corrins that aren't using their autocancels by dashgrabbing as they land.
Autocancelling fair with SHFF on shield could also allow a Corrin to hit shield, land and follow up with a IP with as little as 4 frames of hard landing lag.