decrease vs slow down

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

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

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

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

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