rate vs rapidness

rate and rapidness both are nouns.

Word	Noun	Adjective	Verb	Adverb
rate	Yes	No	Yes	No
rapidness	Yes	No	No	No

As nouns, rapidness is a hyponym of rate; that is, rapidness is a word with a more specific, narrower meaning than rate:

rate (noun)	rapidness (noun)
the relative speed of progress or change	a rate that is rapid
amount of a charge or payment relative to some basis
a quantity or amount or measure considered as a proportion of another quantity or amount or measure
a magnitude or frequency relative to a time unit

rate (verb)	rapidness (verb)
assign a rank or rating to
estimate the value of
be worthy of or have a certain rating