I doubt regular folks can reach 5x or even 2x with good retention. Perhaps if it's a podcast of people chatting about random things, it's possible, but any intellectually stimulating content you probably will have problems. Visually impaired people probably have an advantage on account of having their entire visual cortex freed up and possibly able to help with this (not sure if it does, neuroscience majors here?)

Maybe retention is worse, but I'm still better off. Higher speeds are more engaging and enjoyable to me, as bad speakers and long pauses matter less. Therefore I benefit from watching more talks and lectures. For better retention it is preferable to occasionally re-watch good talks for spaced repetition.

The Video Speed Controller extension is indispensable part of my browsing experience, mostly for keyboard shortcuts to speed up/down and << >> videos.

I used to listen/watch everything at 1.5x-2x speed but found a similar result. Now I only increase the speed if it's information I only need to process once (e.g the news) but not remember later (lectures, tutorials etc).

The problem with retaining is independent of processing information speed.