Google emphasizes the importance of using real user data to evaluate Core Web Vitals instead of relying solely on lab data from tools like PageSpeed Insights (PSI) and Lighthouse.
This reminder comes as Google prepares to update PSI’s throttling settings, which could lead to higher Lighthouse performance scores.
Field Data vs. Lab Data
Core Web Vitals measure a website’s loading speed, interactivity, and visual stability from the user’s perspective.
- Field data reflects real user experiences.
- Lab data is generated in controlled environments using tools like Lighthouse.
Google’s Barry Pollard, a Web Performance Developer Advocate, recently stressed the importance of focusing on field data.
In a LinkedIn post, he advised:
“You should concentrate on your field Core Web Vitals (the top part of PageSpeed Insights) and use the Lighthouse Score only as a rough guide for performance improvements.”
He also highlighted that Lighthouse scores vary significantly based on:
- Where the test is run (e.g., PSI, DevTools, CI).
- Environmental factors that can affect results.
PageSpeed Insights Update
Pollard acknowledged that PageSpeed Insights’ slow servers have caused Lighthouse tests to take longer than expected. To address this, Google is updating PSI throttling settings, which should result in higher performance scores in Lighthouse.
These updates will apply to both the web interface and API, but not other Lighthouse versions.
However, Pollard cautioned:
“A score of 100 doesn’t mean perfect; it just means Lighthouse can’t provide further recommendations.”
The Impact of Goodhart’s Law
Goodhart’s Law states that when a measure becomes a target, it loses its value as a measure. In web performance:
- Focusing only on Lighthouse scores may not improve actual user experience.
- Lighthouse is useful, but it assesses performance in limited conditions.
Alon Kochba, a Web Performance and Software Engineer at Wix, provided additional insight:
“Lighthouse scores may not be the most important – but this is a big deal for Lighthouse scores in PageSpeed Insights.
4x → 1.2x CPU throttling for mobile device simulation, which was off for quite a while.”
Key Takeaway: Optimize for User Experience
As these changes roll out, website owners and developers should prioritize real user experience by focusing on field data for Core Web Vitals.
While Lighthouse scores can guide improvements, they should not be the primary goal.
Google encourages websites to:
- Load quickly
- Respond efficiently
- Maintain visual stability
By prioritizing actual user experience, websites can achieve better long-term performance.
