Watching lectures at 1.5x or 2x: what research says about speed and retention

Summary of published studies and a university press release checked October 30, 2026. We have not run our own experiment. Our editorial method

What the studies found

The most cited study is Murphy, Hoover, Agadzhanyan, Kuehn and Castel (2022), Applied Cognitive Psychology 36(1):69-82, DOI 10.1002/acp.3899. In one experiment, 231 UCLA undergraduates watched two 13 to 15 minute lectures at 1x, 1.5x, 2x or 2.5x, without pausing or taking notes. On the immediate 40-question test, the 1x group averaged 26 correct, the 1.5x and 2x groups about 25, and the 2.5x group about 22. A week later the averages were 24 for 1x, 21 for 1.5x and 2x, and 20 for 2.5x. The lead author summarised that speed had little effect on immediate and delayed comprehension until learners exceeded twice normal speed.

Two smaller studies with medical students point the same way. Kiyak and colleagues (2023, Medical Teacher, DOI 10.1080/0142159x.2023.2189537) randomised 60 students to 1x or 2x: immediate scores were 11.26 versus 10.16 out of 20 and one-week scores 9.66 versus 8.36, differences that were not statistically significant. Merhavy and colleagues (2023, BMC Medical Education, DOI 10.1186/s12909-023-04491-y) compared 1.5x and 2x in 66 students and found no significant difference in quiz scores or a concentration game.

Study	Learners	Speeds	Result
Murphy et al., 2022 (one experiment)	231 undergraduates, two 13 to 15 minute videos	1x, 1.5x, 2x, 2.5x	Immediate: 26, about 25, 25, about 22 of 40. One week: 24, 21, 21, 20 of 40
Kiyak et al., 2023	60 medical students	1x vs 2x	Immediate 11.26 vs 10.16 of 20; one week 9.66 vs 8.36; not significant
Merhavy et al., 2023	66 medical students, one pharmacology lecture	1.5x vs 2x	No significant difference in quiz or concentration measures

How to read these results honestly

Not significant is not the same as zero. In both the Murphy one-week averages and the Kiyak study, the faster groups scored a little lower; the studies were not large enough to show whether gaps of that size are real. If a one- to three-point drop on a 40-point test matters for you, for example before a certification exam, treat 2x as a small risk rather than free time.

The materials were limited. The Murphy lectures were 13 to 15 minutes on the Roman Empire and real-estate appraisal; Merhavy used one pharmacology lecture of about 50 minutes at normal speed and its own limitations section notes it could not confirm students were paying attention. The UCLA team said speeding up may not work as well for especially complex or difficult material. Normal speech runs about 150 words a minute, and the release notes earlier work found comprehension starts to decline as speech approaches about 275 words a minute, near double speed.

Time saved on a 40-hour course

The arithmetic applies to video time only. Reading, quizzes and projects take as long as they take, so check how much of a course is video before counting on these savings.

Speed	Video time for 40 hours of lectures	Hours saved	Evidence on short lectures
1x	40 h 00 min	0	Baseline
1.25x	32 h 00 min	8 h	Not tested separately in these studies
1.5x	26 h 40 min	13 h 20 min	No significant loss versus 1x in Murphy; no significant difference from 2x in Merhavy
1.75x	22 h 51 min	17 h 09 min	Not tested separately in these studies
2x	20 h 00 min	20 h	Small, non-significant drops in Murphy and Kiyak
2.5x	16 h 00 min	24 h	Clear drop in Murphy (about 22 of 40 immediately)

A better use of 2x: watch twice or test yourself

Two of the Murphy experiments are more useful than the headline. Students who watched twice at 2x, which takes the same time as one viewing at 1x, scored the same immediately (25 of 40 for both groups). Students who watched at 2x and then again at 2x a week later outscored a single normal-speed viewing on the delayed test, 24 versus 22.

Murphy's own advice was to spend the saved time on flashcards or practice tests. That matches Roediger and Karpicke (2006), where taking recall tests produced substantially better retention than restudying at delays of 2 days and 1 week, even though restudy felt more reassuring.

Plan: watch at 1.5x or 2x, then close the video and write the three main points from memory before checking.
For a 40-hour course at 2x, reinvest at least half of the 20 saved hours in recall quizzes and a second pass on weak sections.
Rewatch the hardest 10 to 20 percent at 1x rather than the whole course.

When to slow down: checklist

You are typing along with a code, spreadsheet or tool demonstration; pause-and-do beats speed.
The lecture is dense with formulas, proofs or new vocabulary, the case the UCLA authors flagged.
The lecture is not in your first language or the speaker already talks fast; the studies we reviewed did not test second-language listeners.
You cannot write a one-sentence summary after each segment; drop by 0.25x until you can.
The material is for a high-stakes exam; small average drops matter more there.
You are tempted to go above 2x; the one study that tested 2.5x found a clear drop.

Questions before you decide

Does watching lectures at 2x hurt retention?

In the studies we reviewed, 2x produced small drops in average scores that were not statistically significant, while 2.5x produced a clear drop. The lectures were short, so dense or complex courses may behave differently, as the UCLA authors noted.

Is 1.5x speed better than 2x?

Merhavy and colleagues found no significant difference between 1.5x and 2x in 66 medical students, and in Murphy et al. the 1.5x and 2x groups scored about the same. Choose the fastest speed at which you can still summarise each segment from memory.

How much time does 2x save on a 40-hour course?

Forty hours of video takes 20 hours at 2x and 26 hours 40 minutes at 1.5x. Reading, quizzes and projects are not affected, so the real saving depends on how much of the course is video.

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