slow down vs decrease

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

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

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

Other hypernyms of slow down include diminish, fall, lessen.

slow down (noun)	decrease (noun)
	the act of decreasing or reducing something
	the amount by which something decreases
	a change downward
	a process of becoming smaller or shorter

slow down (verb)	decrease (verb)
become less tense, rest, or take one's ease	decrease in size, extent, or range
lose velocity; move more slowly	make become smaller
become slow or slower
reduce the speed of
cause to proceed more slowly