slack off vs let up

slack off and let up both are verbs.

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

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

Other synonyms of slack off include abate, die away, slack.

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