Jehl’s Law of Web Performance

April 21st, 2023. Tagged: performance

Adam Fendrych reported that Scott Jehl said in his Web Expo talk that a website should load before you can say "Cumulative Layout Shift".

screenshot of a tweet

What does that mean in practice?

We're web performance specialists here, we work with measurements and numbers, so we need a more exact number. Numbers reduce ambiguity. To find out that number we need to run the `say` command on a Unix/Mac computer and time the output file.

$ say -o cls.aiff "Cumulative Layout Shift"

Now we open the audio file in music editing software (I'm using Reaper) and observe the timing.
screenshot of a audio file opened in Reaper

One immutable magic number to rule them all

And here we have the answer. A website should load in 1.722 seconds or less. Whatever load means, it's up to you. But yeah, go make it happen!

A website should load before you can say "Cumulative Layout Shift", in other words in 1.722 seconds or less.

Update: CLS

Some overachievers (*cough* Tim Vereecke) are going with the "CLS" abbreviation, instead of the full Cumulative Layout Shift. For these individuals and companies, the magic number is 0.978s or just a second with time to spare. Are you an overachiever?
screenshot of the second audio file opened in Reaper

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