mental deficiency vs slowness

mental deficiency and slowness both are nouns.

Word	Noun	Adjective	Verb	Adverb
mental deficiency	Yes	No	No	No
slowness	Yes	No	No	No

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

Other hypernyms of mental deficiency include backwardness, mental retardation, retardation, subnormality.

mental deficiency (noun)	slowness (noun)
mild mental retardation	a rate demonstrating an absence of haste or hurry
	lack of normal development of intellectual capacities
	unskillfulness resulting from a lack of training