That's my first assumption. It's less strenuous and breathy to sustain an um, at least for me. There's a clear parallel to duh (like uh) usually concerning topi

That's my first assumption. It's less strenuous and breathy to sustain an um, at least for me. There's a clear parallel to duh (like uh) usually concerning topics of [obviousness and stupidity] and hmm (like um) usually concerning topics of [deliberation and intelligence].