Marketing teams spend months on messaging, design and campaigns, then send traffic to pages that make visitors wait.
The 2025 Web Almanac, based on real Chrome user data, found that only 48% of mobile websites pass all three Core Web Vitals. The metric most often failed was largest contentful paint, which only 62% of mobile sites passed (HTTP Archive).
If your site is in the other half, you are paying for traffic that has a worse experience than it should.
The three metrics in plain English
Largest Contentful Paint (LCP): “When can I see it?” How long until the main content, usually the hero image or headline, is visible. Good is 2.5 seconds or less.
Interaction to Next Paint (INP): “Does it respond?” How quickly the page reacts when someone taps, clicks or types. Good is 200 milliseconds or less. INP replaced the older First Input Delay metric in 2024.
Cumulative Layout Shift (CLS): “Does it stay still?” How much the layout jumps around while loading, the thing that makes you tap the wrong button. Good is 0.1 or less.
Google measures these on real visits, at the 75th percentile, so the experience of your slower visitors counts (web.dev).
Why marketers should care
Search ranking is part of the story, but not the main part. The bigger reason is conversion.
In the “Milliseconds make millions” study commissioned by Google and conducted with Deloitte, a 0.1-second improvement in mobile site speed was associated with an 8.4% increase in retail conversions and a 9.2% increase in average order value, as well as lower bounce rates on lead generation pages (web.dev).
Small speed gains add up across every campaign you run.
The fixes that make the biggest difference
Images
The hero image is often the LCP element.
- Serve modern formats such as WebP or AVIF at the size actually displayed.
- Do not lazy-load the hero image; do lazy-load images further down.
- Tell the browser the hero is important, for example with a fetch priority hint.
Third-party scripts
Tag managers, chat widgets, heatmaps, A/B testing tools and ad pixels are the most common cause of slow, unresponsive pages.
- Audit every tag. Remove what nobody uses.
- Load non-essential scripts after the page is usable.
- Be especially careful with anything that runs on every page.
Fonts
- Limit the number of families and weights.
- Preload the one or two fonts used above the fold.
- Use font-display settings that show text immediately.
Layout shifts
- Always set width and height (or aspect ratio) on images, videos and embeds.
- Reserve space for banners, cookie notices and ads.
- Avoid injecting content above what the visitor is already reading.
Heavy interactivity
Long JavaScript tasks hurt INP. Break up heavy scripts, avoid unnecessary frameworks on simple marketing pages and test on mid-range phones, not just the latest laptop.
How AI helps
AI tools are useful in performance work when paired with real measurement:
- summarising performance reports into prioritised fixes;
- identifying which scripts and images are responsible for slow pages across a large site;
- generating optimised image variants and responsive markup;
- reviewing code changes for common performance mistakes.
Always verify improvements with field data from real users, not just a lab score on your own machine.
A monthly routine
- Check Core Web Vitals in Search Console or your analytics.
- Review new tags and plugins added since last month.
- Test your top landing pages on a mid-range phone.
- Fix the worst template first; one template can affect hundreds of pages.
The takeaway
Core Web Vitals are not a technical vanity metric. They describe whether your pages feel fast, responsive and stable to real people, and that shows up directly in conversion.
Performance is built into every website we develop, and it is part of our ongoing website care. For the design side, read the 50-millisecond first impression.
