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UXAtomSpeed

Speed Index (SI)

How quickly the visible part of the page fills in.

Good
3.4 s or less on mobile
Poor
over 5.8 s on mobile

What it measures

Speed Index records the page loading and scores how fast the viewport becomes visually complete. Showing content early scores better than showing it all at the end.

Common causes of a poor SI

  • Everything waits on one large script or stylesheet.
  • Content rendered only by client-side JavaScript.
  • Slow images above the fold.

How to improve it

  1. Server-render the first screen.
  2. Load above-the-fold resources first.
  3. Optimise images in the first viewport.