10 Common Core Web Vitals Issues and Fixes

Want better website performance and higher search rankings? Fix Core Web Vitals issues.

Google uses Core Web Vitals to measure page speed, responsiveness, and visual stability. If your site struggles with slow loading, layout shifts, or poor interactivity, it can hurt user experience and SEO.

Here’s a quick summary of the 10 most common Core Web Vitals issues and how to fix them:

• Slow Server Response: Upgrade hosting, use caching, and deploy a CDN.
• Large Image Files: Resize images, convert to WebP, and enable lazy loading.
• Blocking JavaScript/CSS: Use defer or async attributes and inline critical CSS.
• Heavy JavaScript Load: Split code, reduce unused scripts, and use dynamic imports.
• Main Thread Overload: Break tasks into smaller chunks and use Web Workers.
• Slow Third-Party Code: Audit scripts, load asynchronously, and host critical scripts locally.
• Missing Dimensions: Set explicit width and height for images and ads.
• Layout Shifts from Dynamic Content: Reserve space with CSS placeholders or skeleton UIs.
• Font Loading Delays: Use font-display: swap, preload fonts, and subset characters.
• Poor Resource Optimization: Enable caching, compression, and preload critical assets.

Quick Comparison Table

Core Web Vitals Explained: How To Fix Site Optimization Issues

What Are Core Web Vitals?

Core Web Vitals are metrics that assess a website’s performance in three key areas: page loading speed, interactivity, and visual stability.

• Largest Contentful Paint (LCP): Measures how fast the main content (like an image, video, or block of text) loads. The goal is to achieve LCP in 2.5 seconds or less.
• Interaction to Next Paint (INP): Starting in March 2024, INP will replace First Input Delay (FID). It tracks the delay between user input and visual feedback. Aim for 200 milliseconds or less.
• Cumulative Layout Shift (CLS): Monitors unexpected layout changes during page load. A good score is 0.1 or lower.

Google collects real-user data for these metrics using the Chrome User Experience Report (CrUX). The data is based on a 28-day period and reflects the 75th percentile of performance across mobile and desktop users.

Google Performance Thresholds

Meeting these targets can enhance both user experience and your site’s visibility in search results.

Next, we’ll dive into the ten most common Core Web Vitals problems and how to fix them.

1. Slow Server Response

When your server takes too long to respond, it delays the Time to First Byte (TTFB). This slows down the Largest Contentful Paint (LCP) and delays when users can see important content on your site.

Common Causes

Infrastructure Issues

• Limited server resources like CPU, memory, or bandwidth
• Servers located far from your users
• Overloaded shared hosting environments

Configuration Problems

• Unoptimized database queries
• Missing or poorly configured caching
• Inefficient server-side code
• Plugins or modules that use too many resources

How to Fix It

• Upgrade your hosting: Switch to a VPS or dedicated server that aligns with your site’s traffic and performance needs.
• Use caching: Implement page, object, database query, and browser caching to reduce server load.
• Optimize your database: Clean up redundant data, optimize tables, and add indexes to frequently queried columns.
• Deploy a CDN: A Content Delivery Network (CDN) distributes assets across global servers, reducing latency for users.
• Monitor TTFB: Use tools like Chrome DevTools or WebPageTest to track response times and set alerts for any slowdowns.

Once you’ve addressed server response issues, focus on optimizing large image files to improve your LCP even further.

2. Large Image Files

Large images can slow down your site by increasing download times, delaying page rendering, and negatively affecting user experience.

Common Issues

Oversized Dimensions

• Images that are much larger than their display size.
• High-resolution images unnecessarily used for small thumbnails.
• Serving retina-quality images to all devices, even those without high-resolution screens.

Inefficient Formats

• Using PNG for photos when JPEG would be more efficient.
• Not taking advantage of modern formats like WebP.
• Failing to configure responsive images properly.

Optimization Solutions

• Resize Images to Fit Display Dimensions
  Ensure your images match their intended display size. For example, if your hero image container is 1200px wide, avoid loading a 4000px wide image and scaling it down with CSS. This reduces file size without sacrificing quality.
• Pick the Right Format
  Use JPEG (80-85% quality) for photos, SVG for logos or graphics, and WebP for animations. Always include fallback options for older browsers.
• Use srcset and sizes Attributes
  These attributes allow you to serve appropriately sized images for different screen sizes, improving load times.
• Enable Lazy Loading
  Add the loading='lazy' attribute to defer loading of off-screen images until they’re needed.

Measuring Impact

Check the results of your optimizations using tools like Lighthouse and the DevTools Network panel. These tools can confirm smaller image file sizes and faster Largest Contentful Paint (LCP).

Once you’ve optimized images, you can focus on reducing render-blocking JavaScript and CSS to further enhance loading speeds.

3. Blocking JavaScript and CSS

External CSS and JavaScript files can delay HTML parsing and rendering, which means a slower Largest Contentful Paint (LCP) – sometimes by several seconds, especially on slower connections.

How Blocking Happens

Here’s what goes on behind the scenes:

• The browser starts parsing the HTML until it encounters a CSS or JavaScript file.
• It pauses parsing and rendering to fetch and execute those files.
• Once that’s done, the browser resumes parsing and renders the content.

How to Address This Issue

Blocking scripts and stylesheets directly delay LCP, but you can reduce this impact with the following steps:

• Move nonessential scripts to the footer or load them using defer or async attributes.
• Inline or preload critical CSS for above-the-fold content to prioritize key visual elements.
• Bundle and minify CSS and JavaScript files to reduce the number of requests and file sizes.
• Use <link rel="preconnect"> or <link rel="preload"> tags for important resources to speed up their loading.

Next, we’ll dive into how heavy JavaScript usage can overload the main thread and impact Core Web Vitals even further.

4. Heavy JavaScript Load

Excessive JavaScript execution can significantly harm Core Web Vitals performance, especially after addressing render-blocking scripts.

Impact on Performance

Heavy JavaScript loads create issues like:

• Longer Parsing Times: Large JavaScript files take more time to parse and compile, particularly on mobile devices.
• High Memory Usage: Overloaded scripts can consume a lot of device memory, slowing down the system.
• Main Thread Blocking: Complex scripts can block the main thread, making the page less responsive to user interactions.

Optimization Strategies

Here’s how you can reduce the impact of heavy JavaScript:

• Code Splitting: Break your JavaScript into smaller chunks based on specific routes or features. This ensures only the necessary code loads for the current page, decreasing the initial bundle size and speeding up load times.
• Tree Shaking: Use tree shaking during your build process to eliminate unused code and reduce overall bundle size.
• Dynamic Imports: Load non-critical features dynamically instead of with static imports. For example:

  // Static import: import heavyFeature from './heavyFeature';  // Dynamic import: if (userNeedsFeature) {     const heavyFeature = await import('./heavyFeature'); } 

• Performance Budgets: Set limits for your JavaScript bundle sizes:
  • Main bundle: Max 170KB (minified/compressed)
  • Async chunks: Max 100KB each
  • Total JavaScript: Keep it under 300KB

Monitoring JavaScript Performance

Keep track of your JavaScript performance using these metrics:

• Script parsing time
• Execution duration
• Main thread blocking time
• Bundle size for each route

Integrate automated performance monitoring into your CI/CD pipeline to catch issues before deployment. Regularly audit third-party scripts and remove unused code to maintain peak performance.

Script Optimization Tools

These tools can help you manage and optimize JavaScript loads:

• webpack-bundle-analyzer: Visualizes your bundle composition.
• terser: Minifies your JavaScript files.
• source-map-explorer: Maps and analyzes your code.
• Lighthouse: Provides performance scoring and insights.

5. Browser Main Thread Overload

Even after reducing your JavaScript bundles, long tasks can still clog up the browser’s main thread. This happens when these tasks take over the primary thread, increasing the Interaction to Next Paint (INP) metric and causing delayed responses for users. Too much work on the main thread negatively impacts INP scores, which can hurt your site’s responsiveness and SEO.

How to Fix It

Building on the JavaScript optimizations from Section 4, here’s how you can address main thread overload:

• Move Heavy Processing Elsewhere: Use Web Workers to handle resource-intensive tasks, freeing up the main thread for smoother user interactions.
• Break Down Long Tasks: Split JavaScript tasks into smaller chunks (under 50ms) using tools like requestAnimationFrame or async functions.
• Delay Non-Essential Work: Schedule background tasks during idle browser times with requestIdleCallback or setTimeout.
• Streamline Event Handlers: Keep input event handlers lightweight to reduce processing time.
• Track Task Length: Use the Performance API to identify tasks taking longer than 50ms and break them into smaller pieces.

Measuring Impact

Evaluate your improvements with Chrome DevTools and Lighthouse:

• Check the Long Tasks report.
• Monitor main thread usage.
• Compare INP scores before and after your changes.

Once your main thread is optimized, you can focus on minimizing the impact of third-party code on your Core Web Vitals.

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6. Slow Third-Party Code

Third-party scripts, like analytics tags or social widgets, often come from external domains. These scripts can slow down rendering and interactivity, negatively affecting metrics like LCP (Largest Contentful Paint), INP (Interaction to Next Paint), and CLS (Cumulative Layout Shift). Here are some ways to reduce their impact:

How to Fix It

• Audit and clean up unused tags: Remove outdated analytics, ad, or widget scripts that no longer serve a purpose.
• Use asynchronous loading: Add async or defer attributes to scripts that aren’t critical to the page load.
• Host critical scripts locally: Store essential libraries on your own server to cut down on external requests.
• Preconnect to third-party domains: Use <link rel="preconnect"> to establish early connections to external servers.
• Lazy-load nonessential scripts: Delay loading of social widgets or chat features until after the main content is loaded.

Measuring Impact

Track the effectiveness of these fixes using tools like:

• Chrome DevTools Network panel: Analyze script loading times and identify bottlenecks.
• Lighthouse third-party audit: Get a detailed breakdown of third-party scripts and their impact.
• Core Web Vitals reports in Search Console: Monitor changes in key performance metrics.

Once third-party scripts are optimized, it’s time to ensure all images and ads have properly declared dimensions.

7. Missing Image and Ad Dimensions

If you don’t specify width and height for images and ad slots, browsers allocate space dynamically as elements load. This leads to layout shifts, which can increase your CLS (Cumulative Layout Shift) score and hurt both Core Web Vitals and the user experience.

How to Fix It

• Set explicit dimensions: Add width and height attributes to <img> and ad iframe tags.
• Reserve space for responsive media: Use the CSS aspect-ratio property or padding-based wrappers to ensure space is allocated before content loads.
• Prevent reflows: Apply content-visibility: auto or CSS containment to reduce unexpected layout changes.
• Preload key assets: Use <link rel='preload'> to load dimensioned images early and avoid shifts.

Measuring Impact

• Use Lighthouse or PageSpeed Insights to check for CLS improvements.
• Track the Core Web Vitals report in Google Search Console to monitor reductions in layout shifts.

Once you’ve addressed this, focus on stabilizing layout shifts caused by dynamic content to further improve your pages.

8. Layout Shifts from Dynamic Content

Dynamic elements like ads, embeds, or user-generated content that load late can still disrupt your page layout, even if you’ve set explicit dimensions (as discussed in Section 7). These late-loading elements can push other content around, increasing CLS (Cumulative Layout Shift) scores and undoing earlier optimizations.

To avoid this, you should plan ahead by reserving space for content that loads after the initial page render. This helps prevent sudden, distracting shifts that can harm both user experience and your Core Web Vitals scores.

How to Prevent Layout Shifts

Here are some practical ways to keep your layout stable:

• CSS Placeholder Containers: Use fixed-size wrapper divs with properties like min-height or aspect-ratio to hold space for dynamic content.
• Skeleton UI Components: Display placeholder layouts that mimic the size of the expected content, especially for elements like comment sections or user-generated posts.
• Smart Ad Containers: Set up flexible but size-limited ad containers. This ensures ads of varying sizes fit within a reserved space without causing shifts.
• Loading State Management: Add loading indicators within pre-sized containers to keep the layout consistent while waiting for dynamic content.

Tracking Your Progress

To see if your efforts are paying off, monitor CLS improvements using these tools:

• Real User Monitoring (RUM) data
• Chrome DevTools Performance panel
• Core Web Vitals report in Google Search Console

Next, we’ll dive into font loading delays and how to address them for a more stable layout.

9. Font Loading Delays

Just like blocking scripts and images, delayed fonts can slow down rendering and cause layout shifts. Custom fonts, in particular, might delay text rendering, leading to issues like invisible text or sudden style changes. These problems, known as FOIT (Flash of Invisible Text) and FOUT (Flash of Unstyled Text), can hurt your First Contentful Paint (FCP) and contribute to Cumulative Layout Shift (CLS).

Here are some practical ways to improve font loading:

• Use font-display: swap: This ensures fallback fonts are displayed while custom fonts load.
• Preload key font files: Add <link rel="preload" as="font" type="font/woff2" crossorigin> to prioritize font loading.
• Subset your fonts: Include only the characters you need to reduce file size.
• Convert fonts to WOFF2: This format is lighter and faster to load.
• Host fonts locally: Avoid relying on third-party font services.
• Limit font variations: Stick to essential weights and styles only.

To track your progress, use tools like:

• Lighthouse: Check font loading audit scores.
• DevTools Network panel: Measure font download times.
• Performance recordings: Identify FOIT and FOUT durations.
• Core Web Vitals reports: Look at FCP metrics for insights.

Once you’ve tackled font loading issues, you can focus on optimizing other resources to further improve your Core Web Vitals.

10. Poor Resource Optimization

After addressing server, image, script, and font optimizations, it’s essential to fine-tune resource management to maintain improvements in Core Web Vitals. With fonts loading efficiently, it’s time to focus on resource caching and compression to keep performance on track.

Improving caching, compression, and resource hints can significantly enhance metrics like Largest Contentful Paint (LCP) and Interaction to Next Paint (INP). However, problems often arise due to inefficient caching and compression setups.

Common Caching Issues:

• Missing cache headers for static files
• Incorrect cache duration settings
• Conflicting cache-control directives
• No browser caching in place

Typical Compression Problems:

• Text-based resources not compressed
• Delivery of redundant or unnecessary code
• CSS and JavaScript files left unminified

Solutions to Optimize Resources:

• Set Up Proper Caching
  • Add Cache-Control headers with suitable max-age values.
  • Enable browser caching for static files.
  • Use versioning for cached resources to avoid stale content.
  • Consider using service workers for offline caching.
• Enable Compression
  • Apply Gzip or Brotli compression for HTML, CSS, and JavaScript files.
  • Remove unused code to shrink bundle sizes.
• Use Resource Hints
  • Preload critical assets with <link rel="preload">.
  • Use <link rel="preconnect"> and dns-prefetch for external domains.
  • Prefetch resources for pages users are likely to visit next.

Tools to Monitor and Validate:

• Chrome DevTools: Use the Network panel to confirm caching and compression settings.
• Coverage Tab: Identify and eliminate unused code.
• Performance Panel: Track improvements in load times.
• Lighthouse: Evaluate the impact on Core Web Vitals.

These steps ensure your resources are optimized for better performance and user experience. Up next, a summary of key issues and their fixes.

Issue and Solution Summary

Use this table to quickly identify Core Web Vitals issues and their main solutions:

Regularly audit your site and make updates to ensure metrics like Largest Contentful Paint (LCP), Interaction to Next Paint (INP), and Cumulative Layout Shift (CLS) stay within recommended ranges.

Next Steps

After applying the ten fixes, it’s important to keep your progress steady with regular monitoring and maintenance.

Here’s how to maintain your Core Web Vitals improvements:

• Conduct monthly technical audits using a Core Web Vitals checklist to quickly identify and address any regressions.
• Set up automated real-user monitoring with tools like Chrome UX Report and Search Console alerts to keep an eye on LCP, INP, and CLS in real time.
• Review weekly Core Web Vitals reports to fine-tune performance budgets and update configurations, ensuring your targets stay on track.

Consistency is key to keeping your site optimized.

Related posts

• Core Web Vitals as a Ranking Factor Explained
• How To Test Core Web Vitals With Free Tools
• 10 SEO Metrics for Performance Benchmarking
• Core Web Vitals Metrics Explained: CLS, LCP, INP