mental retardation vs slowness

mental retardation and slowness both are nouns.

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

As nouns, mental retardation and slowness are synonyms defined as:

mental retardation and slowness: lack of normal development of intellectual capacities

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

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