Lighthouse

What is Google Lighthouse?

Lighthouse is an open-source software tool that allows anyone to run web page audits on any webpage online. Lighthouse is used by PageSpeed Insights, GTmetrix, and many other website speed test tools.

Lighthouse tests any webpage on 5 main web page optimization categories:

• Performance (Page experience)
• Accessibility (Assistive technology)
• Best Practices (Web development standards)
• SEO (Technical SEO only)
• Progressive Web App (application)

(This article will only focus on the Performance metric of Lighthouse tests.)

Programatic lab tests

Lighthouse metrics are programatic lab tests that are conducted on your web page to mimic real life scenarios. It's important to note that Lighthouse score are not based on actual user experience.

Instead, lab tests simulate how users may experience your website. Lighthouse provides that simulation.

This varies significantly from field, or actual, data from the Chrome User Experience Report (also known as the Chrome UX Report, or CrUX for short).

Lighthouse performance scores

Lighthouse audits your website and creates a performance score based on key metrics.

At the time of writing this, Lighthouse audits website based on these % weights:

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

You can find a visual overview of this after you run a PageSpeed Insights test on a website. The section section of data on the page shows these values calculated by Lighthouse.

Why do my Lighthouse scores change ?

Lighthouse scores can change based on browser, hardware, network connection, and many other factors.

Lighthouse provides lab data, which tries to mimic real world scenarios. Real world results have many variables and real results usually vary. So, the test is doing its job!

There are so many variables that affect Lighthouse tests and that variability can change your score from test to test.