stick-in-the-mud vs lagger

stick-in-the-mud and lagger both are nouns.

Word	Noun	Adjective	Verb	Adverb
stick-in-the-mud	Yes	Yes	No	No
lagger	Yes	No	No	No

As nouns, lagger is a hypernym of stick-in-the-mud; that is, lagger is a word with a broader meaning than stick-in-the-mud:

Other hypernyms of stick-in-the-mud include dawdler, drone, laggard, poke, trailer.

stick-in-the-mud (noun)	lagger (noun)
someone who moves slowly	someone who takes more time than necessary; someone who lags behind

stick-in-the-mud (adjective)	lagger (adjective)
(used pejoratively) out of fashion; old fashioned