Medium impact · weight 6 · worth up to 2.5 points of the total score
CLS measures content jumping around after it appears. It is why a visitor taps the wrong button when an ad or a late-loading image pushes the page down under their finger.
Reserve space with width and height attributes on images and iframes, and never insert content above what is already on screen.
Check a page for this, and the other 75 checks at once
Interaction to Next Paint
INP is how long the page takes to visibly respond after a tap or a click. Above 200ms the interface starts to feel like it is ignoring the user. INP replaced First Input Delay (FID) as a Core Web Vital in March 2024.
HTML is compressed in transit
Gzip or Brotli typically cuts HTML to a quarter of its size. Serving it uncompressed wastes that bandwidth on every single request, and it is usually one line of server config.
Time to first byte
How long the server took to start answering, measured on our own request. A slow first byte delays everything downstream no matter how well the front end is built.
Response carries cache headers
Without a usable Cache-Control header, browsers and CDNs re-fetch the page on every visit, so returning visitors pay the full load cost again for content that did not change.
