slowness vs backwardness

slowness and backwardness both are nouns.

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

As nouns, slowness and backwardness are synonyms defined as:

slowness and backwardness: lack of normal development of intellectual capacities

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

slowness (noun)	backwardness (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