Lighthouse Performance Score (LPS)

The Lighthouse Performance Score (LPS) is a synthetic metric used to evaluate the speed and performance of websites, provided by the Lighthouse tool. This score is calculated from a series of metrics that simulate a user's page load, offering a rating on speed and content availability.

Included Metrics and Their Weight#

Lighthouse calculates the overall LPS based on several key metrics, each assigned a specific weight reflecting its importance to the user experience.

1. First Contentful Paint (FCP) (weight 10%): Measures the time from navigation to when the browser renders the first bit of content. FCP is crucial for user perception of page loading speed.
2. Speed Index (SI) (weight 10%): Reflects how quickly the page content is visually displayed. Lower values indicate faster loading.
3. Largest Contentful Paint (LCP) (weight 25%): Measures when the largest content element on the page is rendered. LCP is important for assessing when the main content is available.
4. Total Blocking Time (TBT) (weight 30%): Measures the total time the browser's main thread is blocked. TBT is key for evaluating how responsive the page is to user interactions.
5. Cumulative Layout Shift (CLS) (weight 25%): Assesses how often users encounter unexpected layout shifts. CLS is vital for evaluating page stability during loading.

How to Measure It?#

Measuring the Lighthouse Performance Score can be done directly in Chrome DevTools, using PageSpeed Insights, or by integrating Lighthouse CI into development processes. Each of these tools provides a detailed report, including individual metric assessments and improvement recommendations.

Why Lighthouse Score Isn't a True Measure of Web Speed#

A common misconception is equating the Lighthouse score with the actual speed of the entire website or even a specific page.

The Lighthouse score doesn't display real speed. Why?

• It is a synthetic measurement, obtained by machines, not real users.
• You're only measuring one page. If you input a domain, it's the homepage.
• Core Web Vitals metrics aren't accurately calculated by Lighthouse. The layout stability CLS metric is only partially covered, and interaction responses measured in INP metric aren't included at all.
• In tools like PageSpeed Insights, the Lighthouse score is set very strictly, especially within the Central European context.

Consider this practical example of three websites and their vastly different Lighthouse scores:

All three websites have sound speed data from users (CrUX), yet the Lighthouse scores vary significantly.

Avoid using the Lighthouse score to gauge website speed.

Lighthouse Score (LPS) is a metric designed for the speed tuning process. It's advantageous for developers or testers, particularly when optimizing speed or automatically monitoring if the new state isn't worse than the previous one.

The Significance of Synthetic Measurement#

LPS is a synthetic super metric because it comprises measurements from five different metrics.

Regular daily Lighthouse score measurements show the state of a particular aspect of website speed. The advantage of synthetic measurement is that it is cheap and relatively detailed.

In combination with historical data, synthetic metrics can be easily compared and tracked over time. LPS and other synthetic metrics can quickly reveal potential errors occurring during development.

In PLUS monitoring, we also utilize synthetic measurements. The Watchdog Report measures the web daily using Lighthouse and sends alerts if metric limits are exceeded.

Of course, synthetic measurements won't tell you how your website performs speed-wise for your actual users on their devices. For that, you need user measurements (RUM) or CrUX data from Google users, which we also provide.

We discuss the differences between measurements on a dedicated page: synth vs. CrUX vs. RUM.