slow down vs lessen

slow down and lessen both are verbs.

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

As verbs, lessen is a hypernym of slow down; that is, lessen is a word with a broader meaning than slow down:

Other hypernyms of slow down include decrease, diminish, fall.

slow down (verb)	lessen (verb)
become less tense, rest, or take one's ease	decrease in size, extent, or range
lose velocity; move more slowly	wear off or die down
become slow or slower	make become smaller
reduce the speed of
cause to proceed more slowly