retard vs slow down

retard is a noun but slow down is not a noun.

Word	Noun	Adjective	Verb	Adverb
retard	Yes	No	Yes	No
slow down	No	No	Yes	No

As verbs, retard and slow down are synonyms defined as:

Other synonyms of retard include decelerate, slow, slow up.

As verbs, slow down is a hyponym of retard; that is, slow down is a word with a more specific, narrower meaning than retard:

Other hyponyms of retard include slow, slow up, delay, detain, hold up.

As verbs, slow down is a hypernym of retard; that is, slow down is a word with a broader meaning than retard:

Other hypernyms of retard include decelerate.

retard (noun)	slow down (noun)
a person of subnormal intelligence

retard (verb)	slow down (verb)
lose velocity; move more slowly	become less tense, rest, or take one's ease
cause to move more slowly or operate at a slower rate	lose velocity; move more slowly
slow the growth or development of	become slow or slower
be delayed	reduce the speed of
	cause to proceed more slowly