backwardness vs slowness

backwardness and slowness both are nouns.

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

As nouns, backwardness and slowness are synonyms defined as:

backwardness and slowness: lack of normal development of intellectual capacities

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

backwardness (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