Website Speed Optimization Guides

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.

Performance audits: opportunities, diagnostics and field data

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.
• Reduce JavaScript execution time: cut parse, compile and run time per script to lower TBT and INP.
• Minify CSS and JavaScript: ship files without whitespace and comments.
• Reduce DOM size: fewer elements, faster style, layout and interactions.
• font-display and web fonts: keep text visible and load fonts without layout shifts.

Network and third parties

• Enable text compression: Brotli or gzip for HTML, CSS and JavaScript.
• Preconnect and preload: start connections and critical resources earlier.
• Third-party code: what tags, pixels and widgets cost and how to load them without blocking.
• Page weight: how heavy a page is, what is typical and what a budget buys you.

Platforms

• Core Web Vitals for WordPress: pass rates, typical causes and fixes for WordPress.
• Core Web Vitals for Shopify: apps, images and theme: what merchants can fix themselves.
• Core Web Vitals for Next.js: hydration, bundles and image priority inside the framework.

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

• Serve static assets with an efficient cache policy: let returning visitors reuse files instead of re-downloading them.

Core Web Vitals and metrics

Beyond the individual fixes, these guides explain the metrics themselves: what each one measures, what counts as a good score, and which fixes move it.

• Core Web Vitals: what the three metrics are, which values count as good and how Google uses them.
• Core Web Vitals Benchmark: how WordPress, Wix, Squarespace and other platforms perform on Core Web Vitals.
• Lighthouse performance score: how the score is calculated and why it differs from field data.
• Largest Contentful Paint (LCP): how quickly the main content of the page appears.
• Interaction to Next Paint (INP): how quickly the page responds to interactions.
• Cumulative Layout Shift (CLS): how visually stable the page stays as it loads.
• First Contentful Paint (FCP): how soon the first content appears; a supporting loading metric.
• Time to First Byte (TTFB): how long the server takes to start answering; the base of every loading metric.
• Total Blocking Time (TBT): how long long tasks block the main thread; the lab proxy for INP.
• Speed Index: how quickly the visible part of the page fills in during load.

Benchmarks and statistics from PerfDelta's measurements

For the overall picture across every site PerfDelta has measured, from pass rates to page weight and third-party code, see the website speed statistics.

To see how your values compare against sites of the same kind, the industry benchmarks list measured Core Web Vitals by industry, with the platform split inside each one.

Author: Christian Schreiber

The guides are maintained by Christian Schreiber, who built and runs PerfDelta. How the reports measure and rate pages is described on the methodology page.

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.

For a focused check, use the Core Web Vitals test for LCP, INP and CLS with real-user data, or the mobile website speed test for load time on a phone.