Core Web Vitals

Each vital measures something a visitor feels. Largest Contentful Paint times how long the biggest visible element takes to appear. Interaction to Next Paint times how long the page takes to respond when someone clicks or types. Cumulative Layout Shift scores how much the layout moves unexpectedly as the page loads.

Google's guidance is to assess them at the 75th percentile of page loads, split across mobile and desktop, so a page passes only if most real visits were good rather than the average one. That is a different question from what a single lab test answers.

Why it matters for a marketing site

A slow hero or a page that jumps as it loads costs attention before anyone reads a word. The vitals give that a number you can compare across releases. They are not a ranking shortcut: good scores will not lift a page above better content. Treat them as a floor, and as an early warning when a new script arrives.

How I handle it

I run the site performance check before planning a move, so there is a baseline and the third-party scripts slowing the page are named. On this site each page also has a weight budget, and the check fails when a page goes over it. A migration keeps the integrations you need, and those can cost more than the framework saves, so I say which script is costing what rather than promising a score.

Example

Say a homepage's largest element is a hero image that appears at 3.1 seconds on mobile, past the 2.5-second line. Moving it to a modern format, sizing it correctly and loading it first can bring it under. If a tag manager is holding up the main thread instead, the image work will barely register.

Common mistakes

• Reading a single lab score as the truth. A lab run is one simulated visit. Field data shows what real visitors got.
• Testing only on desktop. Google measures at the 75th percentile, split across mobile and desktop, and mobile is usually the harder of the two.
• Blaming the framework first. On many marketing sites the weight is third-party scripts, which come along to any stack.