slowness vs mental retardation

slowness and mental retardation both are nouns.

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

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

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

Other synonyms of slowness include backwardness, retardation, subnormality.

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