slacken vs slow up

slacken and slow up both are verbs.

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

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

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

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