slow down vs diminish

slow down and diminish both are verbs.

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

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

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

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