let up vs slack off

let up and slack off both are verbs.

Word	Noun	Adjective	Verb	Adverb
let up	No	No	Yes	No
slack off	No	No	Yes	No

As verbs, let up and slack off are synonyms defined as:

Other synonyms of let up include abate, die away, slack.

let up (verb)	slack off (verb)
become less in amount or intensity	become less in amount or intensity
reduce pressure or intensity