loosen up vs slow down

loosen up and slow down both are verbs.

Word	Noun	Adjective	Verb	Adverb
loosen up	No	No	Yes	No
slow down	No	No	Yes	No

As verbs, loosen up and slow down are synonyms defined as:

Other synonyms of loosen up include decompress, relax, unbend, unwind.

loosen up (verb)	slow down (verb)
become less tense, rest, or take one's ease	become less tense, rest, or take one's ease
cause to feel relaxed	lose velocity; move more slowly
make one's body limber or suppler by stretching, as if to prepare for strenuous physical activity	become slow or slower
cause to become unblocked	reduce the speed of
become less tense, less formal, or less restrained, and assume a friendlier manner	cause to proceed more slowly