Speed Index is a lab metric that estimates how quickly a page’s visible content is visually displayed during a test run; lower is generally faster.
What Speed Index means
Speed Index is calculated from visual completeness over time in a controlled run. It can help diagnose rendering progress, but it is not a Core Web Vital and does not describe every user’s interaction or field experience. Results vary with the test environment, route, device emulation and page state.
Decision rule and evidence
Use Speed Index to compare like-for-like lab runs and investigate the rendering stages behind a regression. Do not optimise the score in isolation or quote a single test as a universal user experience result.
Practical workflow
- Choose a stable URL, device profile, network setting and cache state.
- Run a baseline and retain the report, filmstrip and test date.
- Locate the visual delay: server response, blocking resource, image, font or client rendering.
- Make one reversible change with a clear hypothesis.
- Repeat the same test and inspect field data separately where available.
Verification and common mistakes
Compare the filmstrip and supporting timings, not just the aggregate score. Mark absent field data as absent; lab improvement does not by itself prove a Core Web Vitals improvement.