Lighthouse: Speed Index

Overview

Speed Index ( SI ) is a performance system of measurement that measures how quickly your page is visually complete above-the-fold .
SI is subject on the size of the browser opinion port and differs from other metrics like First Contentful Paint ( FCP ) or Largest Contentful paint ( LCP ) because it is not a milestone on your page ‘s load timeline .
rather, it is the consequence of a calculation that reflects your visitor ‘s page experience utilizing a frame-by-frame analysis of your page ‘s load behavior .
SI is very much tied to other page load timings. This makes it a useful overall benchmark for assessing your web site ‘s performance in its entirety.

How does your site score on this metric?

What does Speed Index measure?

The Speed Index metric unit is a synthetic time that is calculated, not measured. It is computed using ocular differences between frames captured during your page warhead .
Each ocular frame represents fourth dimension intervals, and each captured frame is scored for visual completeness above-the-fold until all content is visible i.e., when the page is visually complete .

The Speed Index measured measures how visually complete your page is by analyzing its load, frame-by-frame .
The individual scores are then added to compute the page ‘s Speed Index. Click here to learn more about how SI is calculated .
Because other metrics like LCP and FCP affect your page ‘s ocular completeness, SI is a capital way to track their impact in the exalted scheme of things.

think of Speed Index as a unite metric that accounts for many of the early metrics and audits .
SI, on its own, does not give you much actionable data. however, it represents a practical manner to determine if your page rush optimizations are working .

Speed Index’ effect on your Performance Score

Speed Index accounts for 10% of the total Performance Score .
efficaciously, a lower SI indicates that above-the-fold contented loaded much earlier, giving your visitors the perception of a faster load page .

Thresholds for Speed Index

Flickroom displays the calculate Speed Index in seconds, with a lower time representing a faster load web page .
SI thresholds are, as follows :

• Good – nothing to do here = SI of 1311 milliseconds or less.
• OK, but consider improvement = SI between 1311 and 1711 milliseconds.
• Longer than recommended = SI between 1711 and 2300 milliseconds.
• Much longer than recommended = SI higher than 2300 milliseconds.

How to improve Speed Index?

Note that the particular audits mentioned below likely contribute the most to your SI ; however, your foliate ‘s Speed Index may besides be affected by other optimizations that are n’t mentioned hera.

In general, most operation optimizations that result in a debauched First Contentful Paint and Largest Contentful Paint are besides likely to result in a faster Speed Index for your foliate .
Freeing the main-thread for longer, in particular, ensures that the browser can load all visible content faster for your page visitors ( i.e., faster ocular completeness ). Some of the optimizations hera include :
note : Some of the Structure audits are related ; improving one is probable to improve the others .