retardation vs slowing

retardation and slowing both are nouns.

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

As nouns, retardation and slowing are synonyms defined as:

Other synonyms of retardation include deceleration.

retardation (noun)	slowing (noun)
the act of slowing down or falling behind	a decrease in rate of change
a decrease in rate of change
lack of normal development of intellectual capacities
any agent that retards or delays or hinders
the extent to which something is delayed or held back