How-To

How to fix render-blocking resources that slow down your website

March 13, 2025 No comments yet

Render-blocking resources delay a webpage from displaying content quickly by preventing browsers from rendering a page until certain files, like CSS and JavaScript, have fully loaded. These delays lead to poor user experience, lower SEO rankings, and increased bounce rates. Fixing these issues ensures that users see content faster, improving both engagement and search engine performance.

Optimizing render-blocking resources helps:

  • Improve page load speed – Faster load times enhance user experience and reduce frustration.
  • Boost SEO rankings – Google prioritizes fast websites in search results, improving visibility.
  • Reduce bounce rates – Faster websites retain more visitors, increasing engagement and conversions.
  • Enhance Core Web Vitals – Optimizing resource loading improves First Contentful Paint (FCP) and Largest Contentful Paint (LCP), key factors for search ranking.
  • Improve mobile experience – Mobile users, especially those on slower connections, benefit from faster rendering.

Step 1: Identify render-blocking resources

Before optimizing, determine which resources are slowing down your site.

How to find render-blocking resources:

  • Google PageSpeed Insights – Analyze your site and look for “Eliminate render-blocking resources.”
  • GTmetrix – Provides detailed insights on slow-loading CSS and JavaScript files.
  • Chrome DevTools – Open DevTools (F12 or Ctrl + Shift + I), go to the Network tab, and filter for CSS and JavaScript files to check their loading times.
  • Lighthouse audits – Run a Lighthouse audit in Chrome DevTools for a detailed report on performance issues.
  • WebPageTest – Offers advanced resource breakdowns and waterfall views to pinpoint bottlenecks.

Step 2: Optimize CSS delivery

CSS files are often render-blocking because browsers must load and parse them before rendering visible content.

Ways to optimize CSS:

  • Minify CSS files – Use tools like CSSNano, PurifyCSS, or CleanCSS to remove unnecessary characters and whitespace.
  • Use asynchronous loading – Load non-critical CSS asynchronously using media attributes to prevent blocking.
  • Inline critical CSS – Extract above-the-fold styles and insert them directly into the <head> for faster initial rendering.
  • Defer non-critical CSS – Load styles that are not needed for the first render after the page loads.
  • Use a CSS delivery tool – Tools like Critical by Addy Osmani generate optimized CSS delivery strategies.
  • Reduce unnecessary CSS – Audit stylesheets and remove unused styles using tools like UnCSS.

Step 3: Optimize JavaScript execution

JavaScript files can block rendering, especially when they load synchronously.

Ways to optimize JavaScript:

  • Minify JavaScript – Use UglifyJS, Terser, or Closure Compiler to reduce file sizes.
  • Defer JavaScript loading – Add the defer attribute to scripts so they load after the page has rendered.
  • Use async loading – Load JavaScript asynchronously using the async attribute to prevent it from blocking the main thread.
  • Eliminate unused JavaScript – Remove unnecessary scripts or use a modular loading approach.
  • Load third-party scripts wisely – Host critical scripts locally and remove unnecessary external requests.
  • Use code splitting – Break up large JavaScript files to only load necessary code for each page.

Step 4: Optimize web fonts

Custom web fonts can be a major source of render-blocking resources if not properly managed.

Ways to optimize font loading:

  • Use font-display: swap – Ensures text is visible while fonts load asynchronously.
  • Preload key fonts – Add <link rel="preload" href="your-font.woff2" as="font" type="font/woff2" crossorigin="anonymous"> in the <head> to prioritize font loading.
  • Host fonts locally – Avoid slow third-party font services and reduce external dependencies.
  • Use modern formats – Prefer WOFF2 over older formats like TTF or OTF for better compression and performance.
  • Reduce the number of font variations – Limit font weights and styles to only what’s necessary.
  • Subset fonts – Generate a subset of fonts that only include the necessary characters instead of loading full font files.

Step 5: Implement lazy loading for images and iframes

Non-critical images and iframes can delay rendering if not optimized properly.

How to implement lazy loading:

  • Use native lazy loading – Add loading="lazy" to images and iframes to defer loading until needed.
  • Implement JavaScript-based lazy loading – Use libraries like Lozad.js or LazySizes for broader compatibility.
  • Optimize images – Compress images using tools like TinyPNG, ImageOptim, or Squoosh.
  • Use responsive images – Implement srcset to serve different image sizes based on the user’s device.
  • Convert images to modern formats – Use WebP instead of PNG or JPEG for better compression.
  • Eliminate unnecessary images – Remove redundant decorative images that don’t add value to user experience.

Step 6: Utilize a Content Delivery Network (CDN)

A CDN can improve load times by serving assets from geographically distributed servers closer to the user.

Best CDNs for optimizing performance:

  • Cloudflare – Provides free and premium options with automatic performance enhancements.
  • Fastly – Ideal for high-traffic websites requiring real-time caching.
  • BunnyCDN – Affordable, lightweight, and effective for global content delivery.
  • Akamai – Enterprise-grade CDN solutions for large publishers and organizations.
  • StackPath – Easy-to-use CDN with integrated security features.

Step 7: Monitor and test performance improvements

After optimizing render-blocking resources, it’s essential to test your site to ensure changes have a positive impact.

Tools for performance monitoring:

  • Google PageSpeed Insights – Re-run your site audit to measure improvements.
  • GTmetrix – Compare before-and-after results for load speed improvements.
  • WebPageTest – Provides advanced breakdowns of resource loading.
  • Lighthouse reports – Use Chrome DevTools’ Lighthouse audit to identify additional areas for optimization.
  • Real User Monitoring (RUM) – Use RUM tools like New Relic or SpeedCurve to track real-world load performance.

Final thoughts

Fixing render-blocking resources is essential for improving website speed, SEO, and user experience. By optimizing CSS, JavaScript, and fonts, implementing lazy loading, and leveraging a CDN, you can ensure your website loads quickly and efficiently. Regular performance monitoring and adjustments will help maintain and enhance these improvements over time, ensuring that your site remains competitive in search rankings and delivers a seamless user experience.

Michael is the founder and CEO of Mocono. He spent a decade as an editorial director for a London magazine publisher and needed a subscriptions and paywall platform that was easy to use and didn't break the bank. Mocono was born.

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