A Content Delivery Network serves static assets from edge locations close to the user, reducing latency, improving cache hit rates, and offloading traffic from the origin server. Fixing this issue means routing your images, JavaScript, CSS, and fonts through a CDN so visitors anywhere in the world get fast, consistent delivery.

Where to make the change

• Cloud platforms: Cloudflare, Fastly, AWS CloudFront, Bunny.net, and similar providers offer free or low-cost CDN tiers. Sign up, point your domain through the CDN, and enable caching for static asset paths.
• WordPress: use a CDN integration plugin or sign up directly with Cloudflare and update the site's nameservers.
• Shopify: Shopify serves storefront assets through its own CDN automatically; the failure usually points at custom assets uploaded outside the platform.
• Headless or framework sites: deploying to Vercel, Netlify, or Cloudflare Pages puts the entire site behind a global CDN by default.

Common causes and how to resolve them

• No CDN at all: add one. Even a free Cloudflare account in front of an existing host produces measurable speed gains.
• CDN configured but bypassed for some assets: ensure all img, script, link, and font URLs use the CDN hostname rather than the origin.
• Cache headers prevent caching: long-lived assets need cache-control headers like public, max-age=31536000, immutable. Without them, the CDN cannot cache effectively.
• Origin-only third-party scripts: many vendors already serve their scripts via their own CDN. Use the URL the vendor provides rather than self-hosting.

Best practices

• Use a versioned asset path: filenames like app.a3f9.css let you set far-future cache headers without ever serving stale files. Update the filename when content changes.
• Enable Brotli at the edge: most CDNs can compress responses on the fly, including text formats.
• Pre-warm caches for new launches: after a major release, request key URLs through every region (or use the CDN's purge-and-prefetch tools) so users do not hit cold caches.

This test fails when JavaScript or CSS resources block the browser from rendering visible content. Each render-blocking file delays the first paint, and on slower networks the cumulative impact can be severe. Fixing this issue means inlining the truly critical CSS, deferring or asynchronously loading non-essential scripts, and removing unused code.

Example

<head>
  <!-- Inline only above-the-fold critical CSS -->
  <style>/* critical styles here */</style>

  <!-- Load full stylesheet asynchronously -->
  <link rel="preload" href="/styles/main.css" as="style"
        onload="this.onload=null;this.rel='stylesheet'">

  <!-- Defer non-critical scripts -->
  <script src="/scripts/app.js" defer></script>
</head>

Where to make the change

• Application code or templates: identify the CSS and JavaScript needed for the initial render and load only those synchronously. Defer the rest.
• Build pipeline: use a critical CSS extractor (such as critical or penthouse) to generate the above-the-fold styles automatically.
• WordPress: performance plugins offer "delay JS execution" and "load CSS asynchronously" options that handle most of this without code changes.
• Headless or framework sites: use the framework's preload and code-splitting hints to control what blocks rendering.

Common causes and how to resolve them

• Single large stylesheet blocking render: inline critical CSS for the first viewport, load the full stylesheet asynchronously.
• Synchronous third-party scripts in the head: add defer or async, or move them to the end of the body.
• Web fonts blocking text rendering: add font-display: swap in @font-face rules so the fallback font shows immediately.
• Render-blocking @import in CSS: replace with separate <link rel="stylesheet"> tags so the browser can fetch them in parallel.

Best practices

• Use defer for scripts that need the DOM: they execute after parsing but before DOMContentLoaded.
• Use async for fully independent scripts: they execute as soon as they download, in any order.
• Preload the critical request chain: tell the browser early about fonts and key assets with <link rel="preload">.
• Audit with Lighthouse: the Render-blocking resources audit lists every offending file with the estimated time saved by deferring it.