imbecility vs slowness

imbecility and slowness both are nouns.

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

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

imbecility: retardation more severe than a moron but not as severe as an idiot
slowness: lack of normal development of intellectual capacities

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

imbecility (noun)	slowness (noun)
a stupid mistake	a rate demonstrating an absence of haste or hurry
retardation more severe than a moron but not as severe as an idiot	lack of normal development of intellectual capacities
	unskillfulness resulting from a lack of training