What Are Core Web Vitals?
Core Web Vitals (CWV) are a set of metrics developed by Google to measure the actual user experience on web pages. They were announced in 2020 and have been an official Google ranking factor since May 2021. Unlike purely technical metrics like TTFB or Time to Interactive, Core Web Vitals focus on what users actually perceive: loading speed, responsiveness and visual stability.
For Swiss SMEs, CWV are particularly relevant because the Swiss search market is less competitive than the US market – here, technical advantages can make the difference between page 1 and page 2. According to our analyses, over 60% of Swiss SME websites fail at least one Core Web Vitals metric.
The Three Core Web Vitals in Detail
Largest Contentful Paint (LCP) – Loading Speed
LCP measures the point in time when the largest visible element in the viewport has been fully rendered. Typically, this is a hero image, a video poster or a large text block above the fold.
Rating: Good: < 2.5s | Needs Improvement: 2.5s – 4s | Poor: > 4s
Common causes of poor LCP: Slow server response times (TTFB > 800ms), render-blocking JavaScript and CSS, unoptimized or oversized images (the most common problem), client-side rendering without server-side rendering and missing preloading of the LCP resource.
Pro tip: Identify your LCP element in Chrome DevTools (Performance Tab > Timings > LCP). Often it's a hero image. Optimize exactly this element: WebP format, correct size, fetchpriority="high" and <link rel="preload">.
Interaction to Next Paint (INP) – Interactivity
INP replaced FID (First Input Delay) as a Core Web Vital in March 2024. While FID only measured the first interaction, INP captures the latency of all interactions (clicks, taps, keyboard inputs) and reports the 75th percentile value. This is significantly more demanding.
Rating: Good: < 200ms | Needs Improvement: 200ms – 500ms | Poor: > 500ms
Common causes of poor INP: Long JavaScript execution on the Main Thread (Long Tasks > 50ms), third-party scripts (analytics, chat widgets, social plugins), large JavaScript bundles without code-splitting and synchronous DOM manipulations during user interactions.
Pro tip: Use scheduler.yield() or requestIdleCallback() to break long tasks into smaller chunks. Load third-party scripts with defer or async. In our experience, simply loading Google Tag Manager asynchronously improves the INP value by 100–200ms.
Cumulative Layout Shift (CLS) – Visual Stability
CLS measures unexpected layout shifts that occur during the entire page lifecycle. The value is calculated as the product of Impact Fraction (how much of the viewport is shifted) and Distance Fraction (how far the shift goes).
Rating: Good: < 0.1 | Needs Improvement: 0.1 – 0.25 | Poor: > 0.25
Common causes of poor CLS: Images and videos without defined width/height attributes, ads, embeds and iFrames without reserved space, dynamically injected cookie banners or notification bars, web fonts with FOIT/FOUT effects (text jumps when the font loads) and late-loading content above the current viewport.
Pro tip: Use the CSS property aspect-ratio for responsive images and embeds. For web fonts: Use font-display: optional or font-display: swap with <link rel="preload"> for the most important font.
Measuring and Monitoring Core Web Vitals
Lab Data vs. Field Data
There are two types of CWV data: Lab data (synthetic tests under controlled conditions) and Field data (real user data from the Chrome User Experience Report). Google uses exclusively Field data for rankings. Lab data is useful for debugging but not authoritative for rankings.
Google PageSpeed Insights
The free tool shows both lab and field data, provides concrete improvement suggestions with estimated savings potential and is the best starting point for analysis. Test both the mobile and desktop versions of your website.
Google Search Console
The Core Web Vitals report in the Search Console shows how many URLs of your website have good, needs improvement or poor CWV values. The report is based on real user data (28-day rollup) and is the value that Google actually uses for rankings. Prioritize high-traffic URLs first.
Additional Tools
Chrome DevTools: Performance tab for detailed debugging of individual interactions. Lighthouse: Automated audits with actionable recommendations. Web Vitals Extension: Chrome extension that displays CWV in real-time while you browse. CrUX Dashboard: Create a Google Data Studio dashboard with historical CWV data for your website.
Optimization Strategies for Each CWV
Optimizing LCP – The 5 Most Effective Measures
(1) Preload the LCP resource with <link rel="preload">. (2) Use fetchpriority="high" on the LCP element. (3) Optimize images: WebP format, correct size, responsive srcset (more on this in our loading time article). (4) Reduce server response time (TTFB) through CDN and server caching. (5) Eliminate render-blocking CSS and JavaScript.
Optimizing INP – Getting JavaScript Under Control
(1) Identify Long Tasks with Chrome DevTools. (2) Break long JavaScript tasks into chunks under 50ms. (3) Load third-party scripts asynchronously. (4) Implement debouncing and throttling for event handlers. (5) Use Web Workers for computation-intensive tasks off the Main Thread.
Optimizing CLS – Eliminating Layout Shifts
(1) Always define width and height for all images and videos. (2) Reserve space for ads and dynamically loaded content with aspect-ratio or min-height. (3) Preload web fonts and use font-display: swap. (4) Place dynamic UI elements (banners, cookie notices) at the top with position: fixed instead of in the document flow. (5) Avoid inserting content above existing visible content.
CWV and the Swiss Market: Our Experience Values
From over 50 website projects for Swiss SMEs, we have identified the following patterns: The most common CWV problems in Switzerland are unoptimized WordPress themes with too many plugins (LCP problem), cookie banners that load late and cause layout shifts (CLS problem), and Google Tag Manager with too many tags (INP problem).
Websites that we optimized from a CWV score below 50 to 90+ showed an average of 15–25% more organic traffic within 3 months. The bounce rate decreased by 10–20%, and the average session duration increased by 15%.
CWV Optimization: Tools and Workflow
For systematic CWV optimization, we recommend this workflow: First, open Chrome DevTools (F12 → Performance Tab) and run a Lighthouse analysis. This identifies the three biggest performance bottlenecks. Then use the PageSpeed Insights API to compare field data (real users) with lab data (simulated). Lab and field values often don't match – this indicates problems that only occur with real users (e.g., slow mobile devices, unstable connections). For the INP value, the Chrome Web Vitals Extension is particularly helpful: It shows in real-time which interactions take long and which event handler is causing the problem. For CLS debugging, we recommend the Layout Shift Debugger tool in Chrome DevTools, which shows exactly which element shifted and why.
CWV and Mobile-First: Why Mobile Values Decide
Google has used exclusively the mobile version of your website for rankings since Mobile-First Indexing. This means: Your CWV values on mobile are the only ones that matter for SEO. In Switzerland, over 60% of searchers use a smartphone – with significantly weaker hardware than desktop devices. A common mistake: Websites are developed and tested on powerful MacBooks where everything loads fast. On a mid-range Android smartphone with a 4G connection, the result often looks completely different. Always test your website on real mid-range devices or use the CPU throttling feature in Chrome DevTools (4x Slowdown) to simulate realistic conditions.
Core Web Vitals: A Must, Not a Nice-to-Have
Core Web Vitals are more than an SEO ranking factor – they are the benchmark for professional web development. A website with good CWV values is fast, stable and responds immediately to user interactions. This improves not only rankings but also conversion rates, user satisfaction and ultimately your revenue. For Swiss SMEs that want to compete in the digital landscape, optimal Core Web Vitals are the foundation for sustainable online success.
