Google Lighthouse 6 Performance Score Weighting: A Complete Guide

Google Lighthouse


Google Lighthouse is a modern, comprehensive real-world web app performance auditing tool based on Chrome User Experience BigQuery data reports.

Google Lighthouse 6 Release Page | Google Lighthouse 6 Changelog

Changes and updates in Google Lighthouse 6 will be implemented in SpeedFactor in the upcoming weeks.

Google Lighthouse 6 just landed and it measures eight performance-related metrics in the Performance Section of the report with the following Lighthouse performance weighting:

New in Google Lighthouse 6

  • Largest Contentful Paint (LCP) is a new metric that measures the time from navigation until the largest content element in the viewport is rendered (#9905#10213#10452#10529).
    • largest-contentful-paint-element is a companion audit that gives information about which element triggered the LCP (#10517#10713).
  • Cumulative Layout Shift (CLS) is a new metric that measures the amount of unexpected movement of content as a page loads (#9037#10427#10495#10570#10728).
    • layout-shift-elements is another companion diagnostic that gives information about the elements that shifted as the page loaded (#10702).

Google Lighthouse Performance Metrics:

What is First Contentful Paint [FCP]?

First Contentful Paint is the moment when the browser paints first content on the screen.

First Contentful Paint metric insides:

  • First Contentful Paint score has the weight of 20%.
  • The user must see the page ‘appearing’ within the first second after request.
  • The text, logo, icons, canvas or navigation bar contours are considered the FCP.
  • The ‘above-the-fold’ CSS styles are considered critical CSS and must be loaded in the head.

What is First Meaningful Paint [FMC]?

First Meaningful Paint is the moment when page’s primary above-the-fold layout change has happened and meaningful content appeared on the screen.

First Meaningful Paint metric insides:

  • First Meaningful Paint score has the weight of 20%
  • Full-width hero images usually take longer to download and render comparing to headline and above-the-fold text.
  • Non-responsive full-width images are one of the most common reasons for the awful First Meaningful Paint delay.
  • First Meaningful Paint metric requires the web fonts loaded.

What is Page Speed Index [PSI]?

Page Speed Index measures how long the device’s browser takes to convert HTML, CSS, JavaScript and media into visual page content.

Page Speed Index metric insides:

  • Page Speed Index score has the weight of 10%.
  • The lower PSI time, the faster the page loads.
  • The fewer styles and scripts the page content needs the lower PSI is.
  • Page Speed Index depends on user device resources and capabilities.
  • The PSI was initially introduced by WebPageTest.org.
  • The Page Speed Index score is based on Speedline‘s open-source module.

What is Time to Interactive [TTI]?

The Time to Interactive (TTI) is the first moment when page useful content is loaded and is fully interactive to user actions.

Time to Interactive metric insides:

  • The Time to Interactive score has the weight of 15%.
  • TTI requires First Contentful Paint to interact with.
  • TTI is defined when a 50-millisecond interaction response is available.
  • TTI is defined when all event handlers are registered for most visible page elements.
  • This metric was previously called Consistently Interactive.
  • TTI is often affected by JavaScript execution time.
  • The excessive JavaScript libraries and third-party scripts are a common reason for high TTI.

Time to Interactive grades:

  • 0 – 2.2 seconds – ‘excellent’ TTI score.
  • 2.2 – 5 seconds – ‘fast’ TTI score.
  • Over 7 seconds – ‘slow’.

What is First CPU Idle [FCI]

First CPU Idle defines the moment the device’s CPU takes an idle state and awaits a user input.

First CPU Idle metric insides:

  • First CPU Idle score had the weight of 13%.
  • First CPU Idle time is heavily dependent on user device resources and capabilities.
  • First CPU Idle moment occurs when the user can start to interact with the page; TTI occurs when the user is fully able to interact with the page.
  • Since First CPU Idle moment is nearly identical to the Time to Interactive moment, the First CPU Idle metric has been deprecated in Lighthouse 6.

First CPU Idle grades:

  • 0 – 3 seconds – ‘excellent’ CPU Idle score.
  • 3 – 4.7 seconds – ‘fast’ CPU Idle score.
  • Over 6.5 seconds – ‘slow’ score.

What is Max Potential First Input Delay [FID]?

Max Potential First Input Delay audits estimated input latency during the busiest 5-second window of page load. FID measures how fast the app responds to a user input.

Max Potential First Input Delay metric insides:

  • The Max Potential First Input Delay score has no assigned performance score weight.
  • The Max Potential FID is measured within first 5 seconds after Time to Interactive moment.
  • FID is measured as a time needed to process the user input event, execute JavaScript and physically paint the new pixels on the screen.
  • The RAIL performance model recommends a FID below 100 ms.
  • Google Lighthouse’s Estimated Input Latency expects the FID below 50 ms.
  • The input latency above 50 ms makes your page look laggy for 90% of users.
  • First Input Delay is often affected by complex DOM structure and excessive CSS styles.

What is Total Blocking Time [TBT]?

Total Blocking Time measures the total amount of time the page stays is unresponsive to user inputs.

Total Blocking Time metric insides:

  • The Total Blocking Time metric has a weight of 20% and has been introduced in Lighthouse 6.
  • The TBT is often affected by unnecessary JavaScript loading, parsing, and execution.
  • The TBT also occurs as a result of inefficient JavaScript statements and costly set of DOM queries.
  • The TBT score is compared to TBT times of the top 10,000 sites.

Total Blocking Time grades:

  • 0-300ms TBT time is considered ‘fast’.
  • 300-600 TBT time is considered ‘moderate’.
  • Over 600ms TBT time is considered ‘slow’.

What is the Largest Contentful Paint [LCP]?

Largest Contentful Paint is a moment when the largest above-the-fold content element is visible in the viewport and the page looks useful.

Largest Contentful Paint metric insides:

  • The Largest Contentful Paint metric has been introduced in Lighthouse 6.
  • LCP considers the following page elements: img, image, video, divs with background images, etc.
  • Largest Contentful Paint is a user-centric metric for measuring perceived load speed.
  • Native support for LCP launched in Chrome 77.
  • Lighthouse extracts LCP data from Chrome’s tracing tool.

Added by Ciprian on Tuesday, May 19, 2020 in Blog

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