Was looking through this thread, decided to make a gfy off of a video someone posted in here with this tech being done immediately into an fsmash:
Though it's hard to measure exactly because of 30 FPS, in this gif it appears that Marth is able to start the fsmash animation something like 15-18 frames after he first lets go of the ledge...which is honestly really, really fast.
Normal getup animation is something like 32-35 frames, so this essentially appears to be around twice as fast as a normal getup. And since it initially looks sort of like a normal getup, by the time the opponent realizes what you're doing you could catch them a quick jab, uptilt, or grab before they can really react properly.
I know some ppl might have wrote this off because of how hard it is, but being able to have a grab/hitbox out 21-25 frames after leaving the ledge could really be a game-changer for Marth, or whatever other characters possess it. Human reaction time is around 15 frames, so in general this could offer a close to unreactable getup option if it's really only 15-18 frames, especially since initially it's hard to distinguish from a normal getup.