retardation vs slowness

retardation and slowness both are nouns.

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

As nouns, retardation and slowness are synonyms defined as:

retardation and slowness: lack of normal development of intellectual capacities

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

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