MetricsSI
Speed Index
A lab metric for how quickly the visible parts of a page fill in during load, based on video capture.
- Good
- <= 3.4 s
- Needs work
- 3.4 to 5.8 s
- Poor
- > 5.8 s
Lab data (Lighthouse)
Speed Index records the loading filmstrip and scores how fast pixels above the fold become visually complete. Two pages with the same LCP can have very different Speed Index if one fills in gradually and the other pops in all at once.
It is a Lighthouse lab metric, useful for comparing perceived loading speed between builds, but it is not measured on real users.
Related terms
- CWVCore Web VitalsGoogle's set of three field metrics for loading (LCP), interactivity (INP) and visual stability (CLS) of a page.
- LCPLargest Contentful PaintThe time until the largest visible element in the viewport (image, video or text block) has rendered.
- INPInteraction to Next PaintHow long the page takes to visually respond after a user interacts, measured across the whole visit.
- CLSCumulative Layout ShiftA unitless score for how much visible content unexpectedly moves around while the page loads and runs.
- FCPFirst Contentful PaintThe moment the browser renders the first piece of content: text, an image or a canvas.
- TTFBTime to First ByteThe time from the start of navigation until the browser receives the first byte of the response.
- FIDFirst Input Delay (retired)The delay between a visitor's first interaction and the browser starting to process it. Replaced by INP.
- TBTTotal Blocking TimeThe total time the main thread was blocked by long tasks between first paint and interactivity, measured in the lab.
- TTITime to Interactive (legacy)A lab metric for when a page has rendered and can reliably respond to input quickly.
- Page LoadThe classic load event: everything on the initial page, including images and subresources, has finished loading.
- Route LoadThe loading time of an in-app navigation in a single-page app, where no full page reload happens.
- Long Tasks & LoAFAny piece of JavaScript that occupies the main thread for more than 50 ms, blocking the page from responding.
- Experience ScoreA single 0 to 10 number fastmon derives from LCP, INP, CLS, FCP, TTFB, load time and error rate.
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