Google Performance Scores Update

Google have announced a change to the weightings of the different speed metrics that constitute the performance score in Lighthouse version 8.

In Lighthouse version 6 and 7, the following weightings applied:

• Cumulative Layout Shift - 5%
• First Contentful Paint - 15%
• Largest Contentful Paint - 25%
• Speed Index - 15%
• Time to Interactive - 15%
• Total Blocking Time - 25%

In Lighthouse version 8, the following weightings will apply:

• Cumulative Layout Shift - 15%
• First Contentful Paint - 10%
• Largest Contentful Paint - 25%
• Speed Index - 10%
• Time to Interactive - 10%
• Total Blocking Time - 30%

So, Google are giving more weight to Cumulative Layout Shift (CLS) and Total Blocking Time (TBT), with slight decreases for First Contentful Paint (FCP), First Input Delay (FID), and Speed Index (SI).

Cumulative Layout Shift (CLS), a Core Web Vitals that measures visual stability (how often users experience unexpected layout shifts) sees its weighting increase from 5% to 15%. According to Google, the metric’s significant increase can be attributed its established placement in the Core Web Vitals.

CLS is important because users do not want unexpected movements when interacting with a web page. Unforeseen shifts may cause them to do something they didn’t want to do - predictability is key!

In addition to the weighting updates, score curves for three of the metrics will be updated. FCP and TBT will both be stricter meaning that it will be more difficult to retain the same score for these metrics, whilst changes have been made to CLS too.

Despite this, Google forecasts that the “performance score for the majority of sites will stay the same or improve in Lighthouse 8”:

• ~20% of sites may see a drop of up to 5 points, though likely less
• ~20% of sites will see little detectable change
• ~30% of sites should see a moderate improvement of a few points
• ~30% could see a significant improvement of 5 points or more

Performance scores have long influenced SEO, but they’re playing an increasingly bigger role in determining website rankings.

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