slowness vs retardation

slowness and retardation both are nouns.

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

As nouns, slowness and retardation are synonyms defined as:

slowness and retardation: lack of normal development of intellectual capacities

Other synonyms of slowness include backwardness, mental retardation, subnormality.

slowness (noun)	retardation (noun)
a rate demonstrating an absence of haste or hurry	the act of slowing down or falling behind
lack of normal development of intellectual capacities	a decrease in rate of change
unskillfulness resulting from a lack of training	lack of normal development of intellectual capacities
	any agent that retards or delays or hinders
	the extent to which something is delayed or held back