Six minutes and forty seconds. That is what you save by watching a twenty-minute lecture at one and a half times speed. At double speed you save ten. Over a forty-hour course, double speed hands you back twenty hours, which is why almost everyone who takes online courses has pressed the button. The question this post answers is what those hours cost you. It is written for people who already speed up their lectures and want to know where the line is, not for people who need persuading to try it.
The short answer: less than you might fear at moderate speeds, more than you might hope at double speed and beyond, and the research does not fully agree on where between those the cost begins. The most quoted study comes from UCLA, where researchers had 231 students watch short recorded lectures at normal speed, 1.5x, 2x or 2.5x, then tested them straight away and a week later. Right after watching, scores out of forty were about 26 at normal speed and about 25 at double speed. At 2.5x they fell to about 22. A week later the gap at double speed had grown, to about 24 against 21. The authors summarised it plainly:
Results revealed minimal costs incurred by increasing video speed from 1x to 1.5x, or 2x speed, but performance declined beyond 2x speed. (Murphy et al., Applied Cognitive Psychology)
That is the study most "watch at 2x" articles stop at. It is a good study. It is also one study, with short lectures that the authors themselves describe as not especially complex, and later reviews pooling many studies are more cautious.
The speed rule
The fourth step is the one that makes the rule personal. People differ, material differs, and your attention on a Tuesday night differs from your attention on a Saturday morning. The explain test measures all three at once, in about fifteen seconds.
"But the study said 2x was fine."
I have read that research. Double speed made almost no difference. Why would I slow down?
Because two later reviews looked at many more studies and found more cost. A 2025 meta-analysis in Frontiers in Psychology pooled 12 experiments and found that faster playback raised the mental effort of watching and lowered both retention and the ability to apply what was learned, while noting signs of publication bias that make the exact size uncertain. Another 2025 review of 24 studies, reported here, found a small cost up to 1.5x and a moderate to large one at double speed and above. The UCLA result and these reviews are not contradictions so much as different slices: short, clear lectures cope well with speed; harder material and longer delays expose the cost.
Does your course look more like a short, clear explainer, or like dense technical material you will need in a month? If it is the first, the UCLA numbers are a fair guide and 2x may be fine. If it is the second, the reviews are the better guide.
"At normal speed, my mind wanders."
Slow narration is the reason I speed up. At 1x I start checking email, and then I have learned nothing at all.
That is a real problem, and it is a good reason for 1.25x or 1.5x rather than 1x. But the better fix for wandering attention is not more speed, it is a reason to stay present. In one well-known study of video lectures, students who knew they would be quizzed between segments let their minds wander about half as often and took more notes than students who were not quizzed. You can give yourself the same structure: pause at each section break and answer one question from memory before continuing.
"The course is too long, and I have a deadline."
I need this finished by the end of the month. Watching at normal speed is not an option.
Then speed is the wrong lever to pull first. Cutting what you watch saves more time, at less cost, than watching everything faster: skip what you already know, and skim what does not apply to your job. Our guide to skipping parts of a course without missing what matters covers how to decide. For the parts you keep, the UCLA study has one more useful finding: students who watched at double speed and then watched again at double speed shortly before the test did better than students who had watched once at normal speed a week earlier. Watching twice at double speed was not better in itself; what helped was the second fast viewing close to when the knowledge was needed.
Speed is a setting, not a strategy
The speed button is not good or bad. It is a setting, and like any setting it should change with the material. Moderate speed on familiar content is free time. Double speed on new technical content is a loan you repay later, when you realise you cannot use what you watched. The minutes you save are worth most when you spend a few of them testing yourself, because that is where the learning actually happens.
Sources: Murphy et al., Applied Cognitive Psychology, and UCLA Newsroom | Huang, Du and Yang 2025, Frontiers in Psychology | 2025 review of 24 studies, via Gigazine | Szpunar, Khan and Schacter 2013, PNAS | Playback speed in younger and older adults, PMC
