Some verbs, adverbs, and adjectives can be changed to a noun meaning the quality of being _ (umoja, uzuri). So I would assume that 'upesi' would be the noun to the adjective '-epesi', and in one dictionary a secondary meaning for upesi is, indeed listed as a noun meaning 'speed', but a more modern dictionary didn't. So if it is used after the word "kwa", wouldn't it be the noun, (using/with speed), not the adverb (using/with quickly)? And are there other instances of a 'u-' being placed in front of an adjective that changed it to an adverb instead of to a noun? it seems a bit confusing to me to have this word that starts with a 'u' and ends with and adjective being an adverb instead of a noun...???
you're right, upesi is both a noun and an adverb, depending on the context (noun if accompanied by "kwa"). But in this case they're both being used as adverbs. As an adjective it's used mainly in the context of meaning 'lightweight', so they're not perfect synonyms
I can't really think of any off the top of my head, but there aren't many examples of adjectives that also work as adverbs. This could very well be an outlier