retardation vs slowdown

retardation and slowdown both are nouns.

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

As nouns, retardation and slowdown are synonyms defined as:

Other synonyms of retardation include lag.

retardation (noun)	slowdown (noun)
the act of slowing down or falling behind	the act of slowing down or falling behind
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