- Scott H Young - https://www.scotthyoung.com/blog -

New Research Shows I Was Right About Watching Lectures Faster

A new paper argues that watching video lectures at 2x the speed has minimal costs to comprehension. From the abstract [1]:

“We presented participants with lecture videos at different speeds and tested immediate and delayed (1 week) comprehension. Results revealed minimal costs incurred by increasing video speed from 1x to 1.5x, or 2x speed, but performance declined beyond 2x speed. … increasing the speed of videos (up to 2x) may be an efficient strategy, especially if students use the time saved for additional studying or rewatching the videos…”

When I was doing the MIT Challenge [2], people often scoffed at my strategy of watching lectures at 2x (or even 3x speed). This enabled me to watch all the lectures for a full-semester class in as little as two days. Yet, I frequently heard from onlookers that this would make comprehension impossible.

However, now that podcasts are popular, it’s become common knowledge that you can listen at an accelerated pace without suffering significant comprehension losses. Still, it was nice to see research addressing this question with academic materials.

Speculations on Why This Strategy Works

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Speeding up lectures doesn’t seem to severely impact comprehension, yet speed reading probably doesn’t work [3]. What’s the difference?

For starters, lectures proceed at a fixed pace. Reading has always been variable speed—you speed up when your comprehension is high and slow down when it is low. Speed reading advocates claim that you can force the pace higher than feels comfortable, but this involves a comprehension trade-off. The higher speed might be beneficial when you only need the gist, but in those cases it’s better to call it skimming rather than reading.

For speaking, however, our ability to comprehend is generally much faster than a comfortable speaking pace. Additionally, generating speech is probably more demanding than listening. The public speaking advice to speak slower is generally true: if you speak slower, you can be more articulate and careful in your choice of words which makes you sound smarter. But take a recording of that speech, and you can probably speed it up with minimal consequences.

Why Do Hard Classes Feel Too Fast?

If this is true, why did so many people balk at my original suggestion? I think the issue with hard classes isn’t the rate of speech but the amount of background you need to already have to get a full understanding. Hard classes are hard because the speaker glosses over elements that are important for understanding, creating inferential gaps. Knowledgeable students keep pace without problem, but poorer students get lost.

Unfortunately, if there are inferential gaps, simply slowing down the rate of speech may not help much. If you don’t understand, you need more explanation, not the same patchy explanation done more slowly.1 [4]

The irony is that, without a good explanation, chances are you would need the lecture much slower than normal speed to understand. The Feynman Technique [5] is a tool that can help fill in gaps, but this is generally only useful when the gaps are minimal. If you have no understanding whatsoever, it’s usually more efficient to seek alternate explanations of the same idea.

Still, more than ten years after the MIT Challenge, I’m pleased to see research bear out something that I felt intuitively had to be right.


  1. A major exception to this is if the language of instruction is not your native language. While comprehending fluent speech is generally very low cognitive load for native speakers, it can be considerable for those who have it as a second language. I would never dream of watching instruction in Spanish or Chinese at 2x speed. At the same time, when you fail to understand a lecture in your native language, it is almost always because you’re missing knowledge that is assumed by the speaker.