LHS/CWV Performance Runbooks

A set of runbooks to address web page performance issues and follows Lighthouse Scoring (LHS) & Core Web Vitals (CWV) best practices.

The key metrics

Core Web Vitals (CWV)

The three metrics Google uses to assess real-user experience in the field:

• Largest Contentful Paint (LCP)
• Interaction to Next Paint (INP)
• Cumulative Layout Shift (CLS)

Other Web Vitals

Diagnostic and lab metrics that help you understand and improve the Core Web Vitals:

• Time to First Byte (TTFB)
• First Contentful Paint (FCP)
• Total Blocking Time (TBT) (lab proxy for INP)
• Speed Index (SI)

Retired metrics

• First Input Delay (FID): replaced by INP as a Core Web Vital in March 2024, and fully retired in September 2024. Support for it was removed in web-vitals v5.
• Time to Interactive (TTI): removed from Lighthouse in version 10. Use TBT and INP instead.

Use-case runbooks

The metric runbooks above tell you which metric is failing; the use-case runbooks pick up from there with per-root-cause fixes — when each applies, when to skip it, concrete code, and the anti-patterns to avoid:

• Layout stability — shifting banners, embeds, sticky headers, JS reveals (CLS)
• Images — space reservation and LCP hero delivery (CLS, LCP)
• Fonts — font-display, size-adjusted fallbacks, formats, when (not) to preload (CLS, LCP, FCP)
• Critical rendering path — render-blocking resources, request chains, preload/preconnect (FCP, LCP)
• JavaScript cost — long tasks, code splitting, unused code, facades (TBT, INP, LCP)
• Third parties — deferral safety, consent gating, tag containers (LCP, INP, TBT, CLS)
• Server response — cache diagnosis, redirects, compression (TTFB, LCP)

Related performance topics

• Marketing Technologies (MarTech) 3rd-party libraries
• Single-Page Applications (SPA) & soft navigations

Measuring tools

Follow the steps in the performance audit walkthrough, or use one of the tools below.

Lab

• PageSpeed Insights
• Lighthouse in the Chrome DevTools
• Performance panel in the Chrome DevTools, which now also shows live Core Web Vitals for the current page without needing to record a trace
• DebugBear Website Speed Test
• WebPageTest

Note: the standalone Web Vitals Chrome extension is deprecated; its functionality now lives directly in the DevTools Performance panel.

Field

• Chrome User Experience (CrUX) report, and the related CrUX Vis and CrUX History API for trends over time
• Core Web Vitals report in Google Search Console
• Field data surfaced directly in the DevTools Performance panel (powered by the public CrUX API, shown next to your local metrics)
• Microsoft Clarity
• Adobe RUM
• Your own instrumentation via the web-vitals library (v5+), ideally using the attribution build so field data tells you what caused a bad metric, not just its value

How to prioritize fixes

1. Is your page indexed in Google Search Console?
   • If not, defer the fix
2. Do you have real user monitoring (RUM) data for that page?
   • If not, defer the fix
3. Are the CWV metrics failing for that page?
   • If not, defer the fix
4. (optional) Is the page ranking low and having high search volume?
   • If not, defer the fix

Source: https://searchengineland.com/find-core-web-vitals-opportunities-447421

FAQ

Why does Page Speed matter?

Read:

• https://web.dev/learn/performance/why-speed-matters
• https://www.debugbear.com/blog/why-site-speed-matters

Do lab scores (Lighthouse/PageSpeed) affect my search ranking?

No. Google uses field data (CrUX) for ranking signals, not your Lighthouse score. Lab tools are for debugging and iterating; always validate improvements against real-user data.