slow down vs delay

slow down is not a noun while delay is a noun.

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

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

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

slow down (noun)	delay (noun)
	the act of delaying; inactivity resulting in something being put off until a later time
	time during which some action is awaited

slow down (verb)	delay (verb)
become less tense, rest, or take one's ease	cause to be slowed down or delayed
lose velocity; move more slowly	stop or halt
become slow or slower	slow the growth or development of
reduce the speed of	act later than planned, scheduled, or required
cause to proceed more slowly