Depends on the speaker’s rate of unfamiliar information density.
The more information they present orally per unit time, and the less I am unfamiliar with the info, the slower they need to go. Better pictures / animation massively boosts my rate of information uptake.
E.g., PBS Spacetime. The host is an efficient communicator and talks at a moderate rate but the topic is usually some super freaky physics stuff. And the diagrams are ok but not amazing. So I find myself playing 1x and pausing often as my brain melts.
The Minute Physics guy is usually efficient, a bit fast, but the illustrations are great. So if the topic isn’t blowing my mind too much, 1x no sweat.
Some folks are less efficient and use a lot of words to say hardly anything. Some talk slowly, stumble, even pause a lot. Those I run at 1.5x or even 2x (especially if inefficient and slow) or else it drives me nuts with impatience.