I would guess it's more likely that, rather than taking longer to speak or understand it, there'd be a high rate of misunderstanding due to the large phonemic i

I would guess it's more likely that, rather than taking longer to speak or understand it, there'd be a high rate of misunderstanding due to the large phonemic inventory. I suppose the idea is that it's possible to express ideas more rapidly due to a higher entropy signal, but it seems unlikely that it's possible for listeners to hear 13 vowels + 12 diphthongs completely distinctly. I know several non-native but fluent English speakers who find it hard to follow all distinctions of English vowels (between 13-17ish, depending on dialect and including diphthongs), which is significantly fewer than Ithkuil proposes. That assumes that anyone actually could come sufficiently close to speaking it, of course; it seems similarly unlikely that speakers could express so many distinct phonemes sufficiently clearly when the total complexity is so far beyond current languages.