millstone vs impediment

millstone and impediment both are nouns.

Word	Noun	Adjective	Verb	Adverb
millstone	Yes	No	No	No
impediment	Yes	No	No	No

As nouns, impediment is a hypernym of millstone; that is, impediment is a word with a broader meaning than millstone:

millstone: (figurative) something that hinders or handicaps
impediment: something immaterial that interferes with or delays action or progress

Other hypernyms of millstone include balk, baulk, check, deterrent, handicap, hinderance, hindrance.

millstone (noun)	impediment (noun)
one of a pair of heavy flat disk-shaped stones that are rotated against one another to grind the grain	any structure that makes progress difficult
any load that is difficult to carry	something immaterial that interferes with or delays action or progress
(figurative) something that hinders or handicaps