Google is doubling down on its push for real-world performance metrics, advising developers to prioritize field data over lab data when evaluating Core Web Vitals. This move coincides with updates to PageSpeed Insights (PSI) throttling settings, which are set to deliver more accurate Lighthouse performance scores.
Why Field Data Matters
Core Web Vitals focus mainly on key aspects of user experience: loading speed, interactivity, and visual stability. While lab data—generated under controlled conditions using tools like Lighthouse—provides valuable insights, it can’t replicate the variability of real-world user experiences.
Field data, on the other hand, reflects actual user interactions and is crucial for understanding how your site performs under real conditions. Google emphasizes that improving the user experience should be the ultimate goal, not simply boosting lab-based scores.
Google’s Guidance
Barry Pollard, a Web Performance Developer Advocate at Google, recently reinforced this message in a LinkedIn post:
“Focus on your field Core Web Vitals… Lighthouse is a GREAT tool, but it can only test certain things under specific conditions. Improving performance is the key, not just improving the score.”
Pollard also pointed out that while lab data is useful for identifying areas to optimize, Lighthouse scores are highly variable, depending on where and how they’re generated.
PageSpeed Insights Update: What’s Changing?
Google is revising the throttling settings in PSI to better align with real-world conditions. These changes, expected to roll out in the coming weeks, will result in higher performance scores for websites when tested in Lighthouse.
Key updates include:
- Adjusted CPU throttling for mobile device simulations, improving accuracy.
- Faster testing times to address user concerns about slow PSI servers.
- Updates impacting the PSI web interface and API, but not other Lighthouse versions.
Beware of the “Goodhart’s Law” Trap
Pollard referenced Goodhart’s Law, which warns that when a measure becomes a goal, it ceases to be a good measure. In the context of web performance, this means focusing solely on Lighthouse scores might not enhance the actual user experience.
What This Means for Developers
As the PSI update rolls out, Google encourages developers to:
- Use field data to measure real user experience and prioritize improvements based on this data.
- Leverage Lighthouse as a diagnostic tool, but avoid treating its scores as the ultimate goal.
- Focus on delivering fast, responsive, and visually stable websites that delight users.
Bottom Line
While PageSpeed Insights adjustments will bring more realistic Lighthouse scores, the primary takeaway remains clear: prioritize real-world performance and user satisfaction over chasing perfect lab scores. Improving Core Web Vitals isn’t about gaming metrics—it’s about crafting a better web experience for everyone.
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