slow down vs slacken

slow down and slacken both are verbs.

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

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

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

slow down (verb)	slacken (verb)
become less tense, rest, or take one's ease	become looser or slack
lose velocity; move more slowly	make slack as by lessening tension or firmness
become slow or slower	become slow or slower
reduce the speed of	make less active or fast
cause to proceed more slowly