Most of you probably heard that Fox falls the fastest early on, but Falco and Captain Falcon exceed him after some point.
To give you a quick understanding of how falling speed over time works for all the characters, I created this diagram:

This falling speed is applied whenever you are airborne. This is why light fastfallers like Fox and Falco die from the top blast zone so late – falling speed decreases knockback during the entire hitstun and beyond.

This diagram does not account for:
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