The meta-analysis showed that increasing playback speed had increasingly negative effects on test performance. At speeds of up to 1.5x, the cost was very small. But at 2x and above, the negative effect was moderate to large.

To put this in context, if the average score for a cohort of students was 75% with a typical variation of 20 percentage points in either direction, then increasing the playback speed to 1.5x would bring down the average person’s result by 2 percentage points. And increasing the playback speed to 2.5x would lead to an average loss of 17 percentage points.

However, we don’t yet know whether you can reduce the negative effects of fast playback by doing it regularly. So it could be that younger adults simply have more experience of fast playback and are therefore better able to cope with the increased cognitive load. Similarly, it means we don’t know whether younger people can mitigate the negative effects on their ability to retain information by using faster playback more often.

Another unknown is whether there are any long-term effects on mental function and brain activity from watching videos at increased playback speeds. In theory, such effects could be positive, such as a better ability to handle increased cognitive load. Or they could be negative, such as greater mental fatigue resulting from increased cognitive load, but we currently lack the scientific evidence to answer this question.