dullness vs tediousness

dullness and tediousness both are nouns.

Word	Noun	Adjective	Verb	Adverb
dullness	Yes	No	No	No
tediousness	Yes	No	No	No

As nouns, tediousness is a hyponym of dullness; that is, tediousness is a word with a more specific, narrower meaning than dullness:

dullness (noun)	tediousness (noun)
lack of sensibility	dullness owing to length or slowness
without sharpness or clearness of edge or point
the quality of lacking interestingness
a lack of visual brightness
the quality of being slow to understand