Website Performance Audits Explained

Run any site through PageSpeed Insights or Lighthouse and you get a list of recommendations, often phrased in terms that assume you already know what they mean. These guides explain each recommendation in plain language: what the audit is actually measuring, why it affects your loading speed and Core Web Vitals, and the practical steps to fix it, with code examples you can use.

Each guide is written for the person who just saw the warning and wants to resolve it, whether you work on the code directly or manage the site through a CMS. Where an audit connects to another, the guides link across so you can follow the whole thread rather than fixing one issue in isolation.

How performance audits work

Most recommendations fall into two groups. Opportunities are changes that would make the page load faster, such as deferring a script or shrinking an image. Diagnostics describe conditions worth improving, such as a weak cache policy. Both come from a single lab test run in a controlled environment, which is why a fix shows up immediately in the tool but takes longer to appear in field data, the real-user measurements that feed Core Web Vitals over a trailing 28-day window. Fix the lab issue first, then watch the field data confirm it.

JavaScript and CSS

• Eliminate render-blocking resources — clear the CSS and JavaScript that delay the first paint.
• Reduce unused JavaScript — ship less script through code-splitting and tree shaking.
• Reduce unused CSS — strip the stylesheet rules your pages never use.

Images

• Properly size images — serve each image close to the size it is displayed at.
• Serve images in next-gen formats — switch to WebP or AVIF for much smaller files.
• Defer offscreen images — lazy-load below-the-fold images without touching the LCP image.

Server and caching

• Reduce initial server response time — lower your Time to First Byte with caching and a CDN.
• Serve static assets with an efficient cache policy — let returning visitors reuse files instead of re-downloading them.

Test your own site

The fastest way to use these guides is to measure first. Run your URL through PerfDelta to see which of these audits apply to your pages, across mobile and desktop, with lab and field data side by side. Each recommendation in your report links straight to the guide that explains how to fix it.