Great test and demo of Smart Speed by Evan Pederson.
I haven’t done a large-scale test of how much time Smart Speed saves across all known podcasts on average, but on the shows I listen to, it usually averages about 15% faster than normal. I can tweak the parameters to be more aggressive and go a bit faster, but the savings aren’t worth the less-natural pacing. (I listened with Smart Speed full-time myself, trying different values, for almost a year before Overcast’s release.)
I see Smart Speed as getting one more notch on the speed meter for free. Whatever your preferred speed setting is for other reasons (pace, sound quality, intelligibility), Smart Speed tends to get you roughly the actual speed of the next highest one.