A site can look polished, pass a quick desktop test and still frustrate people on ordinary phones and mobile connections. A slow hero image, a delayed cookie banner or a layout that jumps as fonts load can be enough to lose attention before a visitor reads the page. If you want to improve your Core Web Vitals score, treat performance as a product and engineering problem, not a one-off technical SEO task tacked on before launch.
This is for founders, product leads and engineering managers who need their site or app to feel fast for real users, not just score well in a single test. You’ll get a practical breakdown of what each metric measures, how to diagnose the cause rather than the symptom, and how to build a process that keeps scores from sliding back after launch.
What Core Web Vitals actually measure
Google’s Core Web Vitals focus on three parts of page experience: loading speed, responsiveness and visual stability. The metrics are Largest Contentful Paint (LCP), Interaction to Next Paint (INP) and Cumulative Layout Shift (CLS).
LCP measures when the largest meaningful item in the initial view becomes visible, often a hero image, headline block or video poster. A good LCP is 2.5 seconds or less for at least 75% of visits.
INP measures how quickly the page responds visually after someone interacts with it. It replaced First Input Delay because it reflects responsiveness throughout a visit, not only the first click. A good INP is 200 milliseconds or less, and it’s the metric most sites now struggle with, since it demands closer attention to how JavaScript is structured rather than a single quick fix.
CLS measures unexpected movement on the page. A good CLS is 0.1 or below. A button moving just as a visitor tries to press it is not merely an annoyance. It makes the product feel unreliable.
These thresholds are useful guardrails, not the whole story. A landing page with acceptable metrics can still have a poor conversion journey if it asks visitors to wait for a heavy demo, obscures content with pop-ups or makes the next step unclear.
Start with field data, then use lab tools to diagnose
The first mistake is optimising for a single test score. Core Web Vitals assessments use field data collected from real Chrome users over a rolling period, where enough data exists. This is often called CrUX, or real-user data, and it’s what Google Search Console reports against.
Field data tells you whether a problem exists and which pages or device groups are affected. It can reveal, for example, that mobile LCP is poor on a service page while desktop performance is healthy. It cannot always tell you precisely which request, script or component caused the delay.
That is where lab testing helps. Browser developer tools and synthetic performance tests let engineers throttle the network, inspect a performance trace and identify the blocking work. Use both sources together: field data to choose priorities, lab data to reproduce and fix the cause.
Be careful with averages. The relevant question is not whether the average visit is fast, but whether most real visitors get an acceptable experience. Segment by page template, device type, country and connection quality where possible. One oversized asset used across a high-traffic template can affect far more people than an isolated slow page. This kind of prioritisation is exactly what belongs in the technical SEO checklist we run on every launch, rather than being treated as an afterthought once rankings start to slip.
Improve Core Web Vitals score by fixing LCP first
For marketing sites, LCP is commonly the clearest place to begin because the cause is often visible in the first screenful. The usual culprit is not that the site needs a clever optimisation trick. It is that the browser is being asked to download, execute or wait for too much before it can render the main content.
Make the primary content available early
The LCP element should be easy for the browser to discover in the initial HTML. If it is a hero image, use a correctly sized modern image format, provide responsive image variants and avoid loading it through a client-side carousel or delayed JavaScript effect.
Do not lazy-load the image that represents the main content above the fold. Lazy loading is valuable for images further down a page, but applying it to the hero tells the browser to postpone the very asset visitors are waiting for.
Server rendering is often a sensible choice for public-facing pages built with React or Next.js. It allows the initial content to arrive as HTML rather than requiring a device to download and run a large application before showing the page. That does not mean every component must be server-rendered. Interactive sections may need client-side code, but they should not hold up the first view unnecessarily.
Reduce time spent before rendering
Slow server response can consume a significant part of the LCP budget. Review database calls, API dependencies, redirects, cache behaviour and deployment geography. A page that waits on several services before returning HTML will remain slow even if every image is well compressed.
Caching can help, but it needs thought. Content that changes rarely is a good candidate for caching at the edge or generating ahead of time. Account-specific dashboards are different: they may need fresh data and authenticated requests. The practical goal is to cache what is safe to cache, without serving stale or incorrect information.
Third-party scripts are another frequent source of delay. Analytics, consent platforms, chat widgets, A/B testing tools and embedded media can all compete for bandwidth and main-thread time. Keep the services that answer a real business need, defer those that can wait, and remove those nobody uses. Adding another tag is rarely free — we walked through exactly this kind of trade-off in our write-up on shaving a website from 4 seconds to under 1 second.
Make interactions feel immediate with INP optimisation
INP problems usually come from JavaScript doing too much work on the browser’s main thread. When the main thread is busy parsing a large bundle, recalculating layouts or processing a heavy event handler, it cannot respond promptly to a tap or key press.
This matters particularly for application screens with filters, search, forms, dashboards and rich editors. A simple brochure site may have little interaction, while a SaaS interface can have many possible slow paths. Test the interactions people actually use, not only the homepage menu.
Split code by route and feature so users do not download every part of an application upfront. Delay non-essential modules until they are needed, such as a charting library only after someone opens analytics. Keep event handlers focused, and move expensive calculations away from the interaction where possible.
There is a trade-off here. Excessive code splitting can create a chain of small requests and loading states that make an application feel disjointed. The right boundary depends on what a user is likely to do next. Measure the interaction, then optimise the work that blocks it.
Stop layout shifts before users notice them
Most CLS issues are preventable during design and implementation. The browser needs to know how much space an image, video, advert slot or embedded widget will occupy before it arrives. If it does not, content below shifts down when the asset finally loads.
Set width and height or an aspect ratio for media. Reserve space for consent prompts, banners and embedded content. Avoid injecting new elements above existing content after the page has rendered unless the action was initiated by the user.
Web fonts deserve attention too. A late-loading font can change line lengths and move buttons or headings. Use a sensible font loading strategy, limit the number of font files and weights, and check that fallback fonts have similar proportions. Brand typography matters, but loading six weights for a page that uses two is an avoidable performance cost.
Build a page speed process, not a rescue project
The strongest results come from preventing regressions. Performance should be checked when a new component, campaign page, analytics tag or design change is proposed, not only after rankings or conversions fall. This is the same argument we made in why Core Web Vitals should shape your build, not your launch checklist: treat it as a constraint from day one, and you avoid an expensive rescue project later.
A practical Core Web Vitals checklist for any release process should include four habits:
- Set page-level performance budgets for JavaScript, images and key loading metrics.
- Test representative mobile devices and throttled connections before release.
- Monitor real-user data after release, especially on high-value templates.
- Assign ownership so performance issues are investigated rather than left in a backlog indefinitely.
Accessibility and performance often support each other. Clear semantic HTML usually requires less client-side behaviour. Proper image dimensions help both stable layouts and accessible content. Fewer intrusive overlays make pages easier to use for everyone.
If a site has accumulated years of plugins, tracking tags and patched components, the answer may not be another compression plugin. A focused audit can separate quick wins from structural issues, such as an unsuitable rendering approach or an overgrown front-end bundle — the same process we cover in our guide to running a code audit before you scale. DELLIUX approaches this work by tracing the user-visible problem back to the actual request, component or deployment decision causing it.
A better Core Web Vitals score is worthwhile because it reflects a calmer, faster experience for real people, and it tends to move rankings and conversion rates in the right direction at the same time. Start with the pages that matter most to your business, measure what users actually encounter, and make each change small enough to verify.
If you’d like a second pair of experienced eyes on your performance data and the code paths behind it, book a free consultation and we’ll walk through what’s actually slowing your site down.
