promptness vs rapidness

promptness and rapidness both are nouns.

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

As nouns, rapidness is a hypernym of promptness; that is, rapidness is a word with a broader meaning than promptness:

Other hypernyms of promptness include celerity, quickness, rapidity, speediness.

promptness (noun)	rapidness (noun)
the characteristic of doing things without delay	a rate that is rapid
the quality or habit of adhering to an appointed time