Google Lighthouse 6 Performance Score Weighting: A Complete Guide

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:

First Contentful Paint [FCP] – weight of 20%
First Meaningful Paint [FMP] – weight of 20%
Page Speed Index [PSI]– weight of 10%
Time to Interactive [TTI]– weight of 15%
First CPU Idle [FCI] – weight of 13% [deprecated in Lighthouse 6]
Max Potential First Input Delay [FID] – no weight specified
Total Blocking Time [TBT] – weight 20% [introduced in Lighthouse 6]
Largest Contentful Paint [LCP] [introduced in Lighthouse 6]

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 the 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 the 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.