Metrics
Long Tasks & LoAF
Any piece of JavaScript that occupies the main thread for more than 50 ms, blocking the page from responding.
While a long task runs, the browser cannot handle clicks, scrolls or paints, which is exactly what a visitor perceives as jank. Long Animation Frames (LoAF) is the newer API that goes further, attributing a slow frame to the script and even the source line responsible.
Long tasks are the raw material behind a poor INP or TBT. Finding and breaking them up, or moving work off the main thread, is the most direct way to make an interface feel fast.
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.
- SISpeed IndexA lab metric for how quickly the visible parts of a page fill in during load, based on video capture.
- 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.
- Experience ScoreA single 0 to 10 number fastmon derives from LCP, INP, CLS, FCP, TTFB, load time and error rate.
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