Is Lighthouse performance score accurate?

The Lighthouse Performance Score should not be treated as a key performance indicator (KPI) for several reasons. While it provides some insight, it has limitations that make focusing on individual metrics more valuable. Here's why:

Firstly, the performance score is derived from a weighted average of performance metrics within Lighthouse. Currently, metrics like Total Blocking Time (TBT), Cumulative Layout Shift (CLS), and Largest Contentful Paint (LCP) carry more weight, while metrics like First Contentful Paint (FCP) or Speed Index have less weight. This means that a higher score may not accurately reflect the performance of specific metrics. Consequently, fast websites could have low scores, and slow websites could have high scores.

Secondly, the overall performance score lacks actionable insights. If the score decreases, it is unclear how to respond and which aspects need improvement. Relying on a weighted average figure can obscure more practical insights that can guide performance optimization.

Instead, it is crucial to examine individual metrics to gain a clear understanding of your website's performance issues. For example, consider the following metrics highlighted in the green box in the screenshot: LCP, CLS, and FID. These metrics offer a more accurate picture of your site's performance compared to the performance score.

Moreover, it's important to note that Google does not utilize the performance score when evaluating a website. Google's Core Web Vitals, which include LCP, CLS, and FID, are the metrics they consider. Google's John Mueller explicitly stated that "Google doesn’t use the X/100 lighthouse score for search, we use the core web vitals separately (LCP, CLS, FID)."

To summarize, relying on a score that may present a distorted representation of website performance, hinder practical insights, and is not considered by Google is counterproductive. To truly enhance performance, it is essential to gain a nuanced understanding through individual metrics.