rustiness vs slowness

rustiness and slowness both are nouns.

Word	Noun	Adjective	Verb	Adverb
rustiness	Yes	No	No	No
slowness	Yes	No	No	No

As nouns, slowness is a hypernym of rustiness; that is, slowness is a word with a broader meaning than rustiness:

rustiness: ineptitude or awkwardness as a consequence of age or lack of practice
slowness: unskillfulness resulting from a lack of training

Other hypernyms of rustiness include awkwardness, clumsiness, ineptitude, ineptness, maladroitness.

rustiness (noun)	slowness (noun)
ineptitude or awkwardness as a consequence of age or lack of practice	a rate demonstrating an absence of haste or hurry
the condition of being coated or clogged with rust	lack of normal development of intellectual capacities
	unskillfulness resulting from a lack of training