slow up vs decrease

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

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

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

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

slow up (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 up (verb)	decrease (verb)
lose velocity; move more slowly	decrease in size, extent, or range
cause to proceed more slowly	make become smaller
become slow or slower