I think a wrinkle often got lost in the discussion is that the outcome of the learning is less affected by the playback speed but the effort and attention you devote to the learning process. Speedify for the purpose of speedifying may result in a net negative efficiency, but can be used to enable other learning methods, e.g., the "play at 2x so as to study twice" method in the study.

When I was studying for the bar exam, we had handouts and lecture videos in which the lecturer basically read the content out of the handouts ad verbatim. Therefore, technically I won't get anything more by playing the lecture than just reading the handouts. But the handouts are too dry and dull for me to keep focussed in a session, so I played the video at 2x in parallel with my reading. AFAICT the "dual input" did make me learn and remember better. But had I just zoom through the video without the additional efforts of reading, I don't think the result would have been as good.