What is Speed Index in a Lighthouse report?

Speed Index measures how quickly the visible parts of your page fill in as it loads - not just when one element appears, but how fast the whole above-the-fold view becomes complete. A lower number is better:

• Good is under 3.4 seconds in Lighthouse.
• It captures the whole picture filling in - a page that paints content steadily scores better than one that shows nothing then dumps everything at once.
• It is a lab metric - Speed Index comes from Lighthouse’s filmstrip of the load, so it is useful for diagnosis, not a ranking signal itself.

Speed Index rarely needs fixing on its own - it improves as a side effect of better LCP and fewer render-blocking resources. Treat it as a thermometer, not a to-do item: fix the named warnings above it and Speed Index follows.