In this sentence from Orberg LLPSI Cap XLIII: Ab legatis Albanis segnius res acta est it seems that segnius can only be a comparative adverb With the Albani legation the thing moved more slowly. However I can’t see any reference to an advert segnius. L&S gives segniter (adv.) which would seem to bear out my view.
Segnis goes like fortis. Adverb fortiter, comparative adverb fortius, superlative adverb fortissime. So segnis → segniter, segnius, segnissime. The comparative adverb goes like the n. acc. comparative of adjectives. (like altior (m,f) altius (n.) from altus (‘‘high’’).
Thanks for this.