Glossary · Performance
Cumulative Layout Shift (CLS).
A score for how much the page moves around while it loads. Under 0.1 is “good”. It is the reason you tap a link and land on something else.
Why it matters.
The causes are predictable: images without width and height set, fonts swapping in and reflowing the text, banners and cookie notices injected above existing content. All are avoidable at build time and awkward to retrofit, which is why it is worth getting right when a site is built rather than after.
How to check it yourself.
Load your page and try to tap the first link the moment it appears. If the page moves under your thumb, you have a layout shift problem, and so does every visitor.
Questions
Asked often.
How do I check cumulative layout shift (cls) on my own site?
Load your page and try to tap the first link the moment it appears. If the page moves under your thumb, you have a layout shift problem, and so does every visitor.
Two minutes
Where does your
own site land?
Everything above came from opening real websites. Yours takes ten seconds: we load it the way a customer would and say what we find, including when the honest answer is that nothing needs doing.
Start with a
straight answer.
Describe the business and where it works. We come back with what we would change, what it would cost and how long it takes — or with a reason not to bother.