The Coach

The Coach analyses your web page and tells you what to fix. It covers Core Web Vitals (LCP, CLS, INP, FCP), modern image best practices (decoding="async", lazy loading, AVIF/WebP, LCP fetchpriority) and the privacy and security headers that matter today (CSP, COOP/COEP/CORP, Permissions-Policy, X-Content-Type-Options, NEL, Reporting-Endpoints, Referrer-Policy).

The Coach is built into sitespeed.io — when you run sitespeed.io, the Coach runs with it. You can also use the analysis library, coach-core, directly from your own code.

• Introduction — what the Coach checks and how scoring works.
• How to use the Coach — get advice from sitespeed.io or coach-core.
• Developers — write or improve a Coach rule.