delay vs slow down

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

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

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

Other hypernyms of delay include decelerate, retard, slow, slow up.

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

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