Exceedingly slow narration, particularly what's normal for audiobooks, is annoying to me because it's slower than I process words. It's like walking with someone whose pace is far slower than your natural gait -- it takes more energy and concentration to slow down. It's why slow-talkers are so annoying.

This isn't "how fast can I go through this" but "what is a comfortable pace"?

So I bump the speed up, though usually fairly modestly: 1.25x - 1.5x is generally enough.

I've noticed that preferred speeds vary tremendously with the quality of the work and speaker -- high-density information and an exceedingly good speaker, and I'll slow down. Slapdash redundant content and poor speaker, I'll speed up.

The degree of polish in the production matters tremendously. I've listened to CPG Grey's YouTube videos (highly polished) and podcasts (a lot of chit-chat with his co-host). The videos work well at normal speed, or perhaps slighly sped up. The podcasts I find nearly unlistenable, though they improve at much higher speeds (1.75x - 2x).