slow up vs slacken

slow up and slacken both are verbs.

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

As verbs, slow up and slacken are synonyms defined as:

Other synonyms of slow up include slack, slow, slow down.

slow up (verb)	slacken (verb)
lose velocity; move more slowly	become looser or slack
cause to proceed more slowly	make slack as by lessening tension or firmness
become slow or slower	become slow or slower
	make less active or fast