Speed Index
Also called: SI, Lighthouse Speed Index
Speed Index is a lab performance metric that measures how quickly a page's content is visually filled in during load, expressed in seconds. Lighthouse records a video of the page load and scores the visual progression between frames. Lower is better, and it is not a Core Web Vital.
How the score is computed
Lighthouse loads the page, captures a filmstrip video, then passes the frames to the Speedline module, which measures the rate at which pixels get painted. A page that renders most of its content early scores well even if a few late elements trickle in afterward. The metric originated at WebPageTest, and Lighthouse adopted the same approach.
Speed Index is a lab metric. Lighthouse and PageSpeed Insights run it in a controlled environment, so it does not reflect field data from real visitors the way Core Web Vitals do. On mobile the thresholds are:
- 0 to 3.4 s: good (green)
- 3.4 to 5.8 s: needs improvement (orange)
- over 5.8 s: slow (red)
On desktop the good bar is under 1.3 s. In current Lighthouse (v10 through v12) Speed Index carries 10% of the Performance score, so it moves the number less than Total Blocking Time (30%) or Largest Contentful Paint (25%). It tends to track First Contentful Paint and LCP, which means the same fixes help all three: cut render-blocking CSS and JavaScript, trim main-thread work, and set font-display: swap so text does not wait on webfonts before it shows.
How it affects your traffic
Speed Index itself is not a ranking factor (Google ranks on field-measured Core Web Vitals, not lab scores), but a slow one is usually a symptom of the same render-blocking resources and heavy main-thread work that drag LCP and INP down on real devices. Fixing what raises Speed Index (defer scripts, inline critical CSS, compress and lazy-load images) is the same work that lifts the field metrics tied to rankings and cuts bounce on mobile. If your Lighthouse score looks fine but real-user pages still feel slow, that gap is exactly what a Technical SEO engagement diagnoses and closes.
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