Website performance directly impacts revenue, SEO rankings, and user trust. A 100ms improvement in page load time can increase conversions by 1–7%, yet 70% of mobile pages take over 7 seconds to load all content. Use this checklist to audit every layer of your site's performance and build a structured improvement plan.
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Core Web Vitals (2026 Thresholds)
Google's Ranking Signal
Core Web Vitals have been a confirmed Google ranking factor since 2021. INP (Interaction to Next Paint) replaced FID as the responsiveness metric in March 2024.
| Metric | What It Measures | Good | Needs Work | Poor | Primary Tool |
|---|---|---|---|---|---|
| LCP | Largest Contentful Paint - loading speed of main element | < 2.5s | 2.5–4.0s | > 4.0s | PageSpeed Insights |
| INP | Interaction to Next Paint - responsiveness to input | < 200ms | 200–500ms | > 500ms | Chrome DevTools |
| CLS | Cumulative Layout Shift - visual stability | < 0.1 | 0.1–0.25 | > 0.25 | Lighthouse |
LCP
- What It Measures
- Largest Contentful Paint - loading speed of main element
- Good
- < 2.5s
- Needs Work
- 2.5–4.0s
- Poor
- > 4.0s
- Primary Tool
- PageSpeed Insights
INP
- What It Measures
- Interaction to Next Paint - responsiveness to input
- Good
- < 200ms
- Needs Work
- 200–500ms
- Poor
- > 500ms
- Primary Tool
- Chrome DevTools
CLS
- What It Measures
- Cumulative Layout Shift - visual stability
- Good
- < 0.1
- Needs Work
- 0.1–0.25
- Poor
- > 0.25
- Primary Tool
- Lighthouse
Pre-Audit Toolbox
- Google PageSpeed Insights (pagespeed.web.dev) - free, uses real CrUX data alongside lab data
- Lighthouse CLI - run in CI/CD pipelines for automated regression testing
- WebPageTest (webpagetest.org) - waterfall chart, filmstrip view, multi-location testing
- Chrome DevTools → Performance tab - trace JS execution, layout thrashing, long tasks
- Chrome DevTools → Network tab - waterfall, cache analysis, request prioritisation
- Screaming Frog - bulk crawl site for redirect chains, oversized images, missing tags
- GTmetrix - historical tracking, scheduled monitoring, video playback of page load
Server & Loading Performance Checklist
Image & Asset Optimisation Checklist
JavaScript & CSS Optimisation Checklist
Third-Party Script Audit
Third-party scripts are often the biggest performance risk
Analytics, chat widgets, ad scripts, and social embeds can add 500ms–3s to TTI. Audit all third-party scripts quarterly.
Performance Monitoring Setup
- 1
Set up Google Search Console for CrUX data
Link Search Console to Google Analytics 4. Navigate to Core Web Vitals report to see real-user (field) data vs lab data.
- 2
Configure Lighthouse CI in your pipeline
Install @lhci/cli and add a lighthouse CI step in GitHub Actions or your CI tool. Set budgets and fail builds on regressions.
- 3
Install Web Vitals library for Real User Monitoring
Use the web-vitals npm package to capture CWV metrics from real users and send to your analytics platform.
- 4
Set up Sentry Performance or Datadog RUM
These tools capture long tasks, slow transactions, and UI slowdowns from real-user sessions - surfacing issues that lab tests miss.
What’s next?