On the other hand, if you watch speedrun records on YouTube or watch the Summoning Salt documentaries about them, expert speedruns (especially those that allow glitches) are often 10% or less the time that an ordinarily skilled player might take to finish the game. For example, the Super Mario Bros. record is under five minutes, while the Castlevania record is under 11.5 minutes.

I don't think the fact that developers are surprised by speedrun times means that a game is necessarily bad! (And even many of the most extraordinarily beloved games have glitches, often because realistic physics simulation is hard.)

I think I saw a video of Bennett Foddy in which he noted that other people could complete "Getting Over It with Bennett Foddy" considerably faster than he could, and maybe faster than he imagined anyone would be able to.