This test fails when one or more images on the page are served at dimensions significantly larger than the size at which the browser actually renders them. Fixing this issue means resizing the source files (or serving responsive variants) so the bytes downloaded match the pixels actually displayed. The result is faster page loads, lower data usage, and better Core Web Vitals scores.

Example

<img
  src="/images/hero-800.jpg"
  srcset="/images/hero-400.jpg 400w,
          /images/hero-800.jpg 800w,
          /images/hero-1600.jpg 1600w"
  sizes="(max-width: 640px) 100vw, 800px"
  alt="Marathon runners at sunrise"
  width="800" height="450">

Where to make the change

• Raw HTML: use srcset and sizes to let the browser pick the right variant for the user's viewport. Always include intrinsic width and height attributes.
• WordPress: WordPress generates multiple image sizes automatically. Use the the_post_thumbnail() helper or the block editor's image block, both of which emit responsive srcset markup.
• Shopify: use the image_url Liquid filter with size parameters (for example {{ image | image_url: width: 800 }}) and pair with image_tag to emit a responsive image.
• Wix or Squarespace: both platforms generate responsive variants automatically; this issue is typically caused by uploading a deliberately oversized image.
• Headless or framework sites: use the framework's image component (for example next/image or astro:assets), which generates responsive variants and lazy-loads off-screen images.

Common causes and how to resolve them

• Original camera or stock-photo file uploaded as-is: a 6000-pixel-wide JPEG rendered in a 400-pixel column wastes most of the bytes. Resize the source or generate responsive variants.
• No srcset: the browser has no choice but to use the single source. Add multiple variants so mobile users get smaller files.
• Hero images sized for ultra-wide displays: if your largest layout is 1600 pixels, do not ship a 4000-pixel original.
• Background images set in CSS without responsive sources: use the image-set() CSS function or media queries to swap in different image files at different breakpoints.

Best practices

• Generate variants at common widths: 400, 800, 1200, and 1600 pixels covers most viewports cleanly.
• Always set width and height: intrinsic dimensions let the browser reserve space and avoid layout shift, helping CLS.
• Lazy-load off-screen images: add loading="lazy" to images below the fold so the browser only downloads them when needed.
• Pair with modern formats: resizing alone is good; resizing and serving WebP or AVIF compounds the savings.

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 images are served in legacy formats such as JPEG and PNG instead of modern alternatives like WebP or AVIF. Modern formats compress significantly better at equivalent visual quality, which means smaller transfers, faster page loads, and lower data usage for visitors on metered connections. Fixing this issue means generating modern variants and serving them to browsers that support them.

Example

<picture>
  <source srcset="/images/hero.avif" type="image/avif">
  <source srcset="/images/hero.webp" type="image/webp">
  <img src="/images/hero.jpg" alt="Marathon runners at sunrise" width="1600" height="900">
</picture>

Where to make the change

• Raw HTML: use the <picture> element with multiple <source> entries so the browser picks the best supported format.
• WordPress: WordPress 6.x and later supports WebP natively. Use a media optimization plugin to convert existing JPEGs and PNGs and serve modern formats automatically.
• Shopify: Shopify's image CDN can serve WebP automatically; using the image_url filter with format parameter or the image_tag helper handles negotiation.
• Wix or Squarespace: both platforms convert and serve WebP automatically when supported by the visitor's browser.
• Headless or framework sites: components like next/image and astro:assets generate AVIF and WebP variants at build time and serve the best format per request.

Common causes and how to resolve them

• Only legacy formats published: add a build step or image CDN that generates WebP and AVIF alongside the original.
• Single-format <img> tag: wrap the image in <picture> with <source> entries for AVIF and WebP, falling back to the JPEG.
• Image CDN not negotiating format: enable automatic format conversion or pass a format=auto query parameter (depends on provider).
• SVG used where a raster image would be smaller (or vice versa): illustrations and icons should be SVG; photos should be raster in a modern format. Mixing them up wastes bytes.

Best practices

• AVIF first, WebP fallback, JPEG last: AVIF compresses best, WebP is widely supported, JPEG remains the universal fallback.
• Match quality to use case: hero images can use higher quality settings; thumbnails can drop quality without visible loss.
• Use an image CDN: services like Cloudflare Images, Cloudinary, or Imgix handle resizing, format conversion, and caching in one step.

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.