Core Web Vitals: Hidden Issues Killing Your Site Performance [2025 Guide]

Did you know that over 50% of users abandon a website that takes more than 3 seconds to load? Core web vitals have become crucial performance metrics since Google introduced the page experience update in 2020.

Surprisingly, less than 4% of websites successfully pass the Core Web Vitals tests. These metrics measure speed, interactivity, and visual stability of your webpage, directly impacting your site's search rankings. In fact, URLs in position 1 on SERPs are 10% more likely to have good core web vitals scores than those in position 9. Unfortunately, most website owners struggle with core web vitals issues without realizing how they affect their core web vitals SEO performance. Even a minor 0.1-second improvement in page speed can boost conversion rates by up to 8.4%.

In this comprehensive guide, we'll explore the hidden issues damaging your site performance, how to identify them, and provide practical fixes for each core web vital metric. Whether you're dealing with poor LCP (Largest Contentful Paint), struggling with the new INP (Interaction to Next Paint) metric that replaced FID in March 2024, or battling frustrating CLS (Cumulative Layout Shift) problems, we've got you covered.

How Core Web Vitals Affect SEO and User Experience

Core Web Vitals directly influence how Google evaluates and ranks your website, making them crucial for both SEO performance and user satisfaction. When users encounter slow-loading pages or unstable layouts, they're significantly more likely to abandon your site altogether.

Google's 2024 Ranking Signals and CWV

The Page Experience signal remains a key ranking factor in Google's algorithm, combining Core Web Vitals with other user experience metrics. This signal evaluates websites based on specific performance thresholds that reflect real user experiences.

In 2024, Google continues to refine these metrics, with Interaction to Next Paint (INP) officially replacing First Input Delay (FID) in March. INP provides a more comprehensive measurement of responsiveness throughout a user's entire visit, rather than just the initial interaction. The complete phase-out of FID will be finalized by September 2024.

For good performance scores, your site should achieve:

Largest Contentful Paint (LCP): 2.5 seconds or less
Interaction to Next Paint (INP): Under 200 milliseconds
Cumulative Layout Shift (CLS): Less than 0.1

While content quality and relevance remain the primary ranking factors, Core Web Vitals serve as an effective tiebreaker. As Google states, "When content quality is similar, better Core Web Vitals can give a ranking edge". This means that when competing pages offer equally relevant content, the page providing superior user experience will likely rank higher.

Why Core Web Vitals Matter for Bounce Rate and Conversions

The impact of Core Web Vitals extends well beyond search rankings. According to Google's research, websites meeting Core Web Vitals thresholds experience 24% lower abandonment rates. Furthermore, improving page load times by just 0.1 seconds can increase conversion rates by 8% for retail sites and 10% for travel sites.

E-commerce platforms that optimize LCP and CLS have reported up to 15% more conversions. This occurs because users have increasingly high expectations for website performance—they don't just want speed, they demand it. When product images load quickly and interface elements remain stable, users develop greater trust in your site.

Additionally, mobile responsiveness becomes increasingly critical as mobile traffic grows. Mobile users frequently face connectivity challenges and device limitations, making Core Web Vitals optimization particularly important for converting these visitors.

Field Data vs Lab Data: What Google Actually Uses

One common misunderstanding involves which performance data Google actually uses for rankings. To clarify: Google exclusively uses field data (real user measurements) for search rankings, not lab data.

As Google's John Mueller confirmed, "Field data aka CrUX [Chrome User Experience Report] for ranking, not lab tests". This distinction matters because field data represents actual user experiences across various devices, network conditions, and locations over a 28-day rolling window.

Lab data, while valuable for diagnostics, comes from controlled test environments that may not reflect real-world conditions. Tools like Lighthouse generate lab data that can help identify issues but don't directly influence your rankings. Consequently, a perfect Lighthouse score doesn't guarantee SEO success if your field data remains poor.

Barry Pollard, Web Performance Developer Advocate at Google, emphasizes this point: "You should concentrate on your field Core Web Vitals (the top part of PageSpeed Insights), and only use the lab Lighthouse Score as a very rough guide".

Understanding this difference helps prioritize optimization efforts. Focus first on improving metrics that affect real users, then use lab tools to identify and fix specific technical issues.

Hidden Issues Behind Poor LCP, INP, and CLS Scores

Behind every poor Core Web Vitals score lies specific technical issues that silently damage your site performance. Let's examine these hidden culprits that prevent your website from achieving optimal scores.

Unoptimized Images Causing LCP Delays

Largest Contentful Paint (LCP) problems frequently stem from image-related issues. Unoptimized images in outdated formats like PNG can significantly increase load times. In many cases, simply switching to modern formats like WebP or AVIF can reduce file sizes by approximately 50%.

Images without specified width and height attributes create another common problem. When dimensions aren't defined, browsers must recalculate layout space once the image loads, delaying the LCP element's appearance. This issue becomes particularly severe for mobile users or those with slower connections.

Incorrectly implemented lazy loading can also harm your LCP score. While lazy loading benefits below-fold images, applying it to your LCP element (typically above the fold) creates unnecessary delays. Google PageSpeed Insights often flags this with a "Largest Contentful Paint image was lazily loaded" warning.

JavaScript Blocking Main Thread and INP Spikes

The main thread handles most browser operations - rendering HTML, parsing CSS, executing JavaScript, and responding to user interactions. Nevertheless, it can only execute one task at a time. When JavaScript blocks this thread, everything else must wait.

Long-running JavaScript tasks (those exceeding 50ms) are primarily responsible for poor Interaction to Next Paint (INP) scores. These issues manifest through:

JavaScript parsing and execution blocking rendering
Large DOM sizes (exceeding 1,400 nodes)
Complex event callbacks requiring excessive processing
Heavy JavaScript execution after user interactions

Google defines a good INP time as anything below 200ms, with scores above 500ms considered poor. Notably, while nearly all websites passed the old FID metric, data shows only 78.7% have good INP scores, making this a critical area for improvement.

Dynamic Ads and Fonts Triggering CLS Shifts

Cumulative Layout Shift (CLS) issues occur when page elements unexpectedly move during loading. Dynamic advertisements are among the largest contributors to layout shifts. Ad networks often support dynamic ad sizes to increase performance and revenue, however this causes visible content to shift downward, creating a frustrating user experience.

Web fonts represent another common CLS trigger. When downloading and rendering web fonts, browsers typically handle this in one of two ways:

Swapping the fallback font with the web font (Flash of Unstyled Text)
Displaying "invisible" text until the web font becomes available (Flash of Invisible Text)

Both approaches can cause layout shifts as text reflows when fonts are applied. Moreover, embeddable widgets like videos, maps, or social media posts often lack awareness of their content size before loading, causing subsequent layout shifts.

Third-party Scripts Slowing Down Interactions

Third-party scripts impact all three Core Web Vitals metrics. The average web page contains over 35 third-party scripts, with a single script capable of completely blocking your page from rendering. These scripts cause multiple issues:

Network congestion from multiple server requests
Main thread blocking during critical rendering paths
Potential security vulnerabilities from external code
Execution delays affecting user interactions

For INP specifically, third-party scripts cause problems during the initial page load when the main content has mostly rendered but scripts continue blocking the main thread in the background. This creates delays precisely when users begin interacting with your page.

Scripts from analytics, heatmaps, chat services, and advertising platforms can contribute to these issues. Using proper loading strategies like the lazyOnload attribute (especially in Next.js) can minimize their impact by loading scripts during idle time after all resources have been fetched.

How to Identify Core Web Vitals Issues on Your Site

Detecting performance issues early saves time and resources. Several tools can help you pinpoint exactly where your core web vitals need improvement, allowing for targeted optimizations.

Using Google Search Console's CWV Report

Google Search Console offers dedicated Core Web Vitals reports that aggregate real user experiences over a 28-day period. Access these reports through the Experience menu, where you'll find separate assessments for both mobile and desktop performance. The report classifies URLs into three categories: Good, Needs Improvement, and Poor.

What makes this tool valuable is how it groups similar pages together. Instead of analyzing thousands of individual URLs, you can focus on fixing common templates that affect multiple pages simultaneously. For each issue, Search Console provides example URLs and allows you to validate fixes once implemented.

PageSpeed Insights: Field and Lab Data Breakdown

PageSpeed Insights combines two essential data types:

Field data: Real-world performance metrics collected from actual Chrome users visiting your site via the Chrome User Experience Report (CrUX)
Lab data: Simulated tests run in controlled environments to help diagnose specific issues

The field data section shows the 75th percentile of user experiences, making it easier to understand your most frustrating user scenarios. For each metric, PSI displays a distribution across Good, Needs Improvement, and Poor categories.

Remember that Google uses field data exclusively for ranking purposes, not lab data. This distinction helps prioritize optimization efforts toward real-user experiences.

Chrome DevTools for Real-Time Debugging

Chrome DevTools recently introduced a revolutionary feature: real-time Core Web Vitals monitoring directly in the Performance panel. Unlike previous versions that required recording and analyzing after the fact, this panel now shows live metrics as you interact with the page.

This approach solves a fundamental challenge—you no longer need to wait until after recording to discover performance issues. The panel color-codes metrics according to their thresholds, making problems immediately apparent. For deeper analysis, you can examine elements contributing to poor performance, such as seeing which specific element is causing your LCP delays.

Web Vitals Extension for On-the-Fly Checks

The Web Vitals Chrome extension provides instant feedback on Core Web Vitals metrics for any page you visit. This lightweight tool shows color-coded scores for each metric, offering a quick way to spot issues while browsing.

Though currently useful, note that Google plans to end support for this extension on January 7, 2025, as they migrate these features directly into Chrome DevTools. Accordingly, consider transitioning your workflows to the Performance panel for future-proof monitoring.

Fixes for Each Core Web Vital Metric

Optimizing core web vitals requires targeted fixes for each specific metric. Let's explore practical solutions to improve your site's performance scores.

Reducing LCP with Lazy Loading and Image Compression

For images that make up your Largest Contentful Paint element, avoid applying lazy loading as this delays loading your most important visual content. Always specify width and height attributes for images to prevent layout shifts and improve LCP. Modern image formats like WebP or AVIF can reduce file sizes by approximately 50% compared to traditional formats.

If your LCP element is an image, add the fetchpriority="high" attribute to prioritize its loading. This ensures the browser allocates more bandwidth to your critical content. Additionally, remove any data-src attributes that require JavaScript to render images, as 7% of pages obscure their LCP image this way.

Improving INP by Deferring Non-Critical JavaScript

To enhance Interaction to Next Paint scores, break up long JavaScript tasks (those exceeding 50ms) that block the main thread. Implement code splitting to divide large JavaScript bundles into smaller chunks, sending only what's necessary at the beginning.

For non-essential scripts, use appropriate loading strategies:

defer for scripts that need the DOM but aren't immediately required
async for independent scripts like analytics
requestIdleCallback() for lowest-priority tasks that can wait until the browser is idle

Additionally, minimize DOM size and use content-visibility to lazily render off-screen elements, reducing both initial load time and rendering work during user interactions.

Fixing CLS with Reserved Space for Ads and Media

To prevent Cumulative Layout Shift, always reserve space for dynamic content like ads. Add a min-height CSS rule or use the aspect-ratio property to maintain layout stability. For responsive content, use media queries to account for size differences across devices.

Avoid collapsing reserved space even when no content is returned—removing set-aside space causes just as much CLS as inserting content. For web fonts, optimize loading by inlining critical font CSS and using font-display: swap only when fallback and final fonts are similar in size.

Server-Side Rendering and CDN for Faster Load Times

Server-side rendering (SSR) significantly improves Core Web Vitals by delivering fully rendered HTML to browsers. This approach enhances LCP since the largest content element is part of the static content pre-rendered by the server. SSR also reduces layout shifts because browsers don't have to render step-by-step, resulting in better CLS scores.

Lastly, implement a CDN to optimize Time to First Byte by serving content from geographically closer locations. Even caching static HTML documents for a short time can yield performance improvements—yet only 33% of HTML document requests currently utilize CDNs.

Monitoring and Maintaining Core Web Vitals Over Time

Maintaining good core web vitals requires continuous monitoring beyond initial fixes. Establishing proper tracking systems helps you prevent performance regression and sustain search ranking improvements over time.

Setting Up Alerts with Lighthouse CI or Dynatrace

Integrating Lighthouse CI into your development workflow catches performance issues before they reach production. This NodeJS package automatically runs audits on code commits, preventing poor performance from entering your codebase. For comprehensive setups, configure thresholds for each metric and track progress through an SQLite or PostgreSQL database for long-term history.

Dynatrace offers deeper insights with full support for core web vitals metrics. Unlike basic tools that provide aggregated field metrics, Dynatrace lets you drill into details from single instances. For any poorly performing page, you can analyze waterfall charts to identify slow-loading resources and determine whether issues are front-end or back-end related.

Tracking 28-Day Rolling Averages in GSC

Google Search Console evaluates core web vitals performance using a 28-day sliding window. Therefore, after implementing fixes, you'll need patience—it takes approximately one month to see the full impact of your changes. To validate fixes, click "Start Tracking" in the Search Console Core Web Vitals report to begin a 28-day monitoring session.

During this validation period, examine traffic patterns and investigate any metric fluctuations. Even small changes can shift your status from "Poor" to "Needs Improvement" or "Good". Remember that any single URL experiencing issues during this window will mark the problem as unresolved.

Using Vercel Speed Insights for Real-Time Feedback

Vercel Speed Insights provides immediate performance data without waiting for Google's 28-day cycle. This tool collects metrics directly from Chromium browsers as users access your site. Additionally, it allows filtering by:

Device type (mobile/desktop)
Environment (preview/production)
Performance metrics (RES, FCP, LCP)
Time ranges shorter than Google's 28-day window

The dashboard displays three useful views: a time-based line graph showing percentile data, a Kanban board highlighting routes needing improvement, and a geographical map revealing performance by country.

Prioritizing Mobile vs Desktop Performance Separately

Google ranks your site separately for mobile and desktop users based on their respective experiences. Consequently, you should monitor both versions individually. Focus first on mobile optimization as it typically faces more constraints from network limitations and processing power.

When analyzing performance data, examine device-specific patterns that might indicate environment-related issues rather than code problems. Tools like Search Console and Vercel Speed Insights offer dedicated device filtering to help identify these differences.

Conclusion

Core Web Vitals significantly impact both your search rankings and user experience, making them essential performance indicators for any website owner. Throughout this guide, we've explored how these metrics directly affect bounce rates, conversions, and ultimately your bottom line. Field data from real users stands as Google's exclusive ranking signal, not lab data from controlled environments.

Our examination revealed several hidden culprits behind poor performance scores. Unoptimized images often delay LCP, while JavaScript blocking the main thread causes INP spikes. Dynamic ads and web fonts frequently trigger frustrating CLS shifts for users. Third-party scripts simultaneously damage all three Core Web Vitals metrics.

You can now identify these issues using tools like Google Search Console, PageSpeed Insights, Chrome DevTools, and the Web Vitals extension. Each metric requires specific optimization techniques—from proper image handling for LCP to JavaScript deferral for INP and reserved space implementation for CLS. Server-side rendering combined with CDN implementation offers comprehensive improvements across all metrics.

Lasting performance gains demand continuous monitoring. Lighthouse CI alerts, 28-day rolling averages in Search Console, and real-time feedback from tools like Vercel Speed Insights help maintain your improvements. Mobile optimization deserves priority attention due to its stricter performance constraints.

Remember, even small improvements yield significant results. A mere 0.1-second reduction in page load time can boost conversion rates by up to 8.4%. While achieving perfect scores may seem daunting, each optimization step moves you closer to better user experiences and higher search rankings. Your efforts to improve Core Web Vitals will undoubtedly pay dividends through increased engagement, conversions, and ultimately, business success.

FAQs

Q1. What are Core Web Vitals and why are they important? Core Web Vitals are a set of specific factors that Google considers important in a webpage's overall user experience. They measure aspects like loading performance, interactivity, and visual stability. These metrics are crucial because they directly impact your website's search engine rankings and user satisfaction, potentially affecting bounce rates and conversions.

Q2. How can I identify Core Web Vitals issues on my website? You can use several tools to identify Core Web Vitals issues. Google Search Console provides a dedicated report showing real user experiences over time. PageSpeed Insights offers both field and lab data breakdowns. Chrome DevTools now features real-time Core Web Vitals monitoring. Additionally, the Web Vitals Chrome extension can provide quick checks while browsing.

Q3. What are some common hidden issues affecting Core Web Vitals scores? Common hidden issues include unoptimized images causing LCP delays, JavaScript blocking the main thread leading to INP spikes, dynamic ads and fonts triggering CLS shifts, and third-party scripts slowing down interactions. These issues often go unnoticed but can significantly impact your site's performance.

Q4. How can I improve my website's Core Web Vitals scores? To improve Core Web Vitals scores, focus on specific fixes for each metric. For LCP, optimize images and avoid lazy loading critical content. To enhance INP, defer non-critical JavaScript and break up long tasks. For CLS, reserve space for dynamic content like ads and optimize font loading. Additionally, consider implementing server-side rendering and using a CDN for faster load times.

Q5. How long does it take to see improvements in Core Web Vitals after making changes? After implementing fixes, it typically takes about 28 days to see the full impact of your changes in Google's metrics. This is because Google evaluates Core Web Vitals performance using a 28-day sliding window. During this period, it's important to continuously monitor your site's performance and investigate any fluctuations in metrics.