Speed Index

Long definition

Speed Index answers a question single-point metrics like FCP and LCP miss: how fast does the page fill in? Two pages can hit FCP at 1.0s and LCP at 2.5s with completely different in-between experiences — one progressively reveals content frame by frame, the other shows a header and then snaps to fully loaded at 2.5s. Speed Index distinguishes them.

The calculation, originally from WebPageTest and now in Lighthouse: record the page load as video, compute the percentage of "visual completeness" of each frame versus the final state, and integrate the incompleteness over time. A page that's 90% visually complete at 1s and 100% at 2s scores better than one that's 10% at 1s and 100% at 2s. Units are milliseconds, so lower is better.

Google's Speed Index thresholds:

• Good — under 3.4s
• Needs improvement — 3.4s to 5.8s
• Poor — above 5.8s

Weight in the Lighthouse v10+ performance score: 10%. Smaller than TBT (30%) or LCP (25%) but still material.

Speed Index lives in lab only — there's no field version, because frame-by-frame video capture isn't available in the browser performance APIs. It does not appear in CrUX or in the Search Console Page Experience report. Treat it as a diagnostic for the load curve, not a Core Web Vital.

The interventions that move Speed Index: prioritize above-the-fold rendering (inline critical CSS, preload hero image, lazy-load below-fold), avoid layout shifts mid-load (which hurt perceived progress), and serve the HTML fast enough that the visual fill window can start early.

Common misconceptions

• "Speed Index is a Core Web Vital." It isn't. Core Web Vitals are LCP, CLS, INP. Speed Index is a Lighthouse lab metric.
• "Lower Speed Index always means better LCP." Mostly correlated, but not always. A page can score well on Speed Index by filling above-the-fold quickly and still have a large LCP element load late. Each metric measures something different.
• "Speed Index is measured by Google in field data." No. It requires frame-by-frame video and is lab-only. CrUX reports LCP, CLS, INP, FCP, and TTFB — never Speed Index.