As I’m working on De Belville for upcoming performance, I completely agree with you. I think, when sung, it will work much better at a slight slower tempo. For two reasons—1) there are a lot of words, and one needs a bit more time to make them clear and keep the audience engaged; and 2). Sullivan makes his usual mistake when writing for baritones (Pish-Tush, Mr. Goldbury, Ludwig, Giuseppe) of simply not leaving enough time to snatch a real breath! A mildly slower tempo will avoid hyperventilation. Also, there are a few nice harmonic turns in the accompaniment which don’t make their effect if rushed.