retard vs slow up

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

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

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

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

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

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

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

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