slow up vs retard

slow up is not a noun while retard is a noun.

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

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

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

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

Other hypernyms of slow up include decelerate, slow, slow down, slow up.

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

slow up (verb)	retard (verb)
lose velocity; move more slowly	lose velocity; move more slowly
cause to proceed more slowly	cause to move more slowly or operate at a slower rate
become slow or slower	slow the growth or development of
	be delayed