I think you should watch at whatever speed you best ingest the material. Don't get me wrong, some lectures I watch at 2x but it isn't about optimizing speed. It's about optimizing attention. I go faster if the speaker is too slow and/or too monotone so my mind starts drifting. Speeding up can force me to concentrate and not drift but the right speed is dependent in many factors that it can't be a hard rule. Regardless of the speed, in a lecture video I'll pause and rewind. That's the large benefit to them, though at the cost of being unable to engage and interact.

There's no deficit of time in the day, the deficit is in energy and attention. I'm worried we conflate speed and productivity (I'm unsure if you do or don't) but these are original. Speed gives the illusion of productivity but it is ignorant of quality and efficiency. It's only a first order approximation of productivity and a noisy one at best. Velocity is more important, as it requires direction but even velocity is ignorant of momentum, acceleration, force, and many other factors that go into making one productive. Speed is only important if everything else is held constant.

Let's not confuse speed for productivity. I'd argue that is a major contributing factor that has gotten us to this point. Where everything appears to move fast but in reality moves slower than ever