slacken vs slow down

slacken and slow down both are verbs.

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

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

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

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