haven't listened to radiolab in a while so can't remember off the top of my head what speed i'd put it at - but i think the difficulty there with speeding it up is how much they intersperse and cut things in to each other.
I mean, yeah, these are narratives - sometimes i like that and sometimes i just want the juice. If i can't keep up with it i obviously slow it down. If i find my mind has wandered i'll pause/rewind - if that keeps happening i'll stop and come back to it another time cos i'm obviously not giving it enough attention.
I'm not pretending to be a genius but English is native to me and I have tangential knowledge of what they're talking about already often enough so it's rare I have to hang on every word and ponder every implication.
I tend to adjust speed depending on 1) what i'm doing and 2) how noisy it is around me/how well i can focus/hear what they're saying. If i'm not keeping up or it's way too much effort, i slow down. That said, I'm never past 1.5x cos i just don't enjoy straining.