A Technical search engine optimization Information to Lighthouse Efficiency Metrics

A Technical SEO Guide to Lighthouse Performance Metrics

Possibly you’re right here since you’re a die-hard fan of efficiency metrics. Or perhaps you don’t know what Lighthouse is and are too afraid to ask.

Both is a superb choice. Welcome!

For this merry journey into demystifying developer documentation, I’ve recruited Technical search engine optimization and Google Knowledge Studio nerd, Rachel Anderson.

Collectively, we’re hoping to take your efficiency enchancment efforts from “make all of the numbers inexperienced to some clear and significant motion gadgets.

We’re going to take a look at:

  • What the heck is Lighthouse? (In case you didn’t know and have been afraid to ask.)
  • Updates to how Efficiency Rating is calculated (variations 6 and seven).
  • Tips on how to check efficiency utilizing Lighthouse.
  • What metrics comprise Lighthouse’s Efficiency rating.
  • What these metrics imply.
  • Tips on how to enhance them.

What Is Lighthouse?

Lighthouse is an open-source auditing tool that gives standardized scores throughout 5 areas:

  • Efficiency.
  • Progressive Internet App.
  • Finest Practices.
  • Accessibility.
  • search engine optimization.


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For the needs of this text, we’re going to make use of the title Lighthouse to discuss with the collection of assessments executed by the shared Github repo, whatever the execution technique.

Lighthouse runs efficiency assessments utilizing emulated knowledge, also called lab knowledge.

That is efficiency knowledge collected inside a managed surroundings with predefined machine and community settings.

Lab knowledge is useful for debugging efficiency points. It doesn’t imply that the expertise in your native machine in a managed surroundings represents the experiences of actual people within the wild.

Updates to Lighthouse: Internet Core Vitals

On Might 5, 2020, the Chromium undertaking announced a set of three metrics with which the Google-backed open-source browser would measure efficiency.

The metrics, generally known as Internet Vitals, are a part of a Google initiative designed to supply unified steerage for high quality indicators.

The purpose of those metrics is to measure net efficiency in a user-centric method.

Inside two weeks, Lighthouse v6 rolled out with a modified model of Web Core Vitals on the coronary heart of the replace.


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July 2020 noticed Lighthouse v6’s unified metrics adopted throughout Google merchandise with the discharge of Chrome 84.

Chrome DevTools Audits panel was renamed to Lighthouse. Pagespeed insights and Google Search Console additionally reference these unified metrics.

Internet Core Vitals comprise 55% of Lighthouse’s weighted efficiency rating. This variation in focus units new, extra refined objectives.

Total, most pages noticed minimal impression with 83.32% of tests shifting ten factors or much less on the shift to v6.

Model 7 is at present out on Github and was slated for large-scale rollout with the secure Chrome 89 launch in March 2021.

Tips on how to Check Efficiency Utilizing Lighthouse

Methodology Issues

Out of the field, Lighthouse audits a single web page at a time.

A single web page rating doesn’t symbolize your web site, and a quick homepage doesn’t imply a quick web site.

Check a number of web page varieties inside your web site.

Determine your main web page varieties, templates, and purpose conversion factors (signup, subscribe, and checkout pages).

Instance Web page Testing Stock

URL Web page Sort
/ Homepage
/instruments Class Template
/instruments/screwdrivers Product Itemizing Web page Template
/acme/deluxe-anvil Product Element Web page Template
/cart Cart
/checkout Checkout
/order-confirmation Order affirmation
/weblog Weblog Root
/weblog/roadrunners-101 Weblog Template

Earlier than you start optimizing, run Lighthouse on every of your pattern pages and save the report knowledge.

File your scores and the to-do record of enhancements.

Forestall knowledge loss by saving the JSON outcomes and using Lighthouse Viewer when detailed end result data is required.

Get Your Backlog to Chunk Again Utilizing ROI

Getting growth assets to motion search engine optimization suggestions is difficult.

An in-house search engine optimization skilled may destroy their pancreas by having a birthday cake for each backlogged ticket’s birthday. Or not less than be taught to hate cake.

In my expertise as an in-house enterprise search engine optimization professional, the trick to getting efficiency initiatives prioritized is having the numbers to again the funding.

This beginning knowledge will grow to be greenback indicators that serve to justify and reward growth efforts.

Chances are high you’re going to have a couple of space flagged throughout assessments. That’s okay!

If you happen to’re questioning which modifications may have probably the most bang for the buck, take a look at the Lighthouse Scoring Calculator.

Tips on how to Run Lighthouse Exams

This can be a case of many roads resulting in Oz. Certain, some scarecrow is perhaps significantly loud a few sure shade of brick but it surely’s about your objectives.


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Trying to combine search engine optimization assessments into the discharge course of? Time to be taught some NPM.

Have lower than 5 minutes to prep for a potential shopper assembly? A few one-off stories ought to do the trick.

Whichever means you execute, default to cellular until you will have a particular use-case for desktop.

For One-Off Reviews: Chrome Devtools

Check one web page at a time with the Lighthouse panel in Chrome DevTools. As a result of the report will probably be emulating a person’s expertise utilizing your browser, use an incognito occasion with all extensions, and the browser’s cache disabled.

Professional tip: Create a Chrome profile for testing. Maintain it native (no sync enabled, password saving, or affiliation to an present Google account) and don’t set up extensions for the person.

Tips on how to Run a Check Lighthouse Utilizing Chrome DevTools

  1. Open an incognito occasion of Chrome.
  2. Navigate to the Community panel of Chrome Dev Instruments (Command + Choice + I on Mac or Management + Shift + I on Home windows and Linux).
  3. Tick the field to disable cache.
  4. Navigate to the Lighthouse panel.
  5. Click on Generate Report.
  6. Save the file.


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Execs of Working Lighthouse From DevTools

Cons of Working Lighthouse From DevTools

  • One report at a time.
  • Requires you to manually save outcomes.

For Testing the Identical Pages Steadily: net.dev

It’s similar to DevTools however you don’t have to recollect to disable all these pesky extensions!

  1. Go to web.dev/measure/.
  2. Check in utilizing your Google account.
  3. Enter your URL.
  4. Click on Run Audit.

Execs of Working Lighthouse From net.dev

  • Captures a nifty timeline of outcomes! (So long as you’re logged in).

A Technical SEO Guide to Lighthouse Performance Metrics

Cons of Working Lighthouse From net.dev

  • One report at a time.
  • You’ll want to recollect which URLs you’re monitoring over time.

For Testing at Scale (and Sanity): Node Command Line

1. Install npm.
(Mac Professional tip: Use homebrew to keep away from obnoxious dependency points.)


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2. Set up the Lighthouse node module with npm

set up -g lighthouse

3. Run a single textual content with

lighthouse <url>

4. Run assessments on lists of usings by working assessments programmatically.

Execs of Working Lighthouse From Node

  • Many stories will be run without delay.
  • May be set to run routinely to trace change over time.

Cons of Working Lighthouse From Node

  • Requires some coding information.
  • Extra time-intensive setup.

Lighthouse Efficiency Metrics Defined

A Technical SEO Guide to Lighthouse Performance Metrics

In variations 6 and seven, Lighthouse’s efficiency rating is product of seven metrics with every contributing a proportion of the whole efficiency rating.

Metric Title Weight
Largest Contentful Paint (LCP) 25%
Whole Blocking Time (TBT) 25%
First Contentful Paint (FCP) 15%
Velocity Index (SI) 15%
Time To Interactive (TTI) 15%
Cumulative Format Shift (CLS) 5%

Largest Contentful Paint (LCP)

What it represents: A person’s notion of loading expertise.


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Lighthouse Efficiency rating weighting: 25%

What it measures: The purpose within the web page load timeline when the web page’s largest picture or textual content block is seen inside the viewport.

The way it’s measured: Lighthouse extracts LCP knowledge from Chrome’s tracing tool.

Is Largest Contentful Paint a Internet Core Very important? Sure!

LCP Scoring

Purpose: Obtain LCP in < 2.5 seconds.

LCP time

(in milliseconds)

0–2,500 Inexperienced (quick)
2,501-4,000 Orange (reasonable)
Over 4,000 Crimson (gradual)

What Components Can Be A part of LCP?

  • Textual content.
  • Pictures.
  • Movies.
  • Background photographs.


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What Counts as LCP on Your Web page?

It relies upon! LCP usually varies by web page template.

This implies you could measure a handful of pages utilizing the identical template and outline LCP.

Tips on how to Outline LCP Utilizing Chrome Devtools

  1. Open the web page in Chrome.
  2. Navigate to the Efficiency panel of Dev Instruments (Command + Choice + I on Mac or Management + Shift + I on Home windows and Linux).
  3. Hover over the LCP marker within the Timings part.
  4. The ingredient(s) that correspond to LCP is detailed within the Associated Node area.

A Technical SEO Guide to Lighthouse Performance MetricsWhat Causes Poor LCP?

Poor LCP usually comes from 4 points:

  1. Gradual server response instances.
  2. Render-blocking JavaScript and CSS.
  3. Useful resource load instances.
  4. Shopper-side rendering.

Tips on how to Repair Poor LCP

If the trigger is gradual server response time:

  • Optimize your server.
  • Route customers to a close-by CDN.
  • Cache property.
  • Serve HTML pages cache-first.
  • Set up third-party connections early.

If the trigger is render-blocking JavaScript and CSS:


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  • Minify CSS.
  • Defer non-critical CSS.
  • Inline crucial CSS.
  • Minify and compress JavaScript recordsdata.
  • Defer unused JavaScript.
  • Reduce unused polyfills.

If the trigger is useful resource load instances:

  • Optimize and compress photographs.
  • Preload essential assets.
  • Compress textual content recordsdata.
  • Ship totally different property based mostly on the community connection (adaptive serving).
  • Cache property utilizing a service employee.

If the trigger is client-side rendering:

  • Reduce crucial JavaScript.
  • Use one other rendering technique (Try the breakdown of rendering choices within the Guide to Angular).

Assets For Bettering LCP

Whole Blocking Time (TBT)

What it represents: Responsiveness to person enter.

Lighthouse Efficiency rating weighting: 25%

What it measures: TBT measures the time between First Contentful Paint and Time to Interactive. TBT is the lab equal of First Enter Delay (FID) – the sphere knowledge used within the Chrome Consumer Expertise Report and Google’s upcoming Web page Expertise rating sign.


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The way it’s measured: The full time during which the principle thread is occupied by duties taking greater than 50ms to finish. If a activity takes 80ms to run, 30ms of that point will probably be counted towards TBT. If a activity takes 45ms to run, 0ms will probably be added to TBT.

Is Whole Blocking Time a Internet Core Very important? Sure! It’s the lab knowledge equal of First Enter Delay (FID).

TBT Scoring

Purpose: Obtain TBT rating of lower than 300 milliseconds.

TBT time

(in milliseconds)

0–300 Inexperienced (quick)
301-600 Orange (reasonable)
Over 600 Crimson (gradual)

First Enter Delay, the sphere knowledge equal to TBT, has totally different thresholds.

FID time

(in milliseconds)

0–100 Inexperienced (quick)
101-300 Orange (reasonable)
Over 300 Crimson (gradual)

Lengthy Duties & Whole Blocking Time


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TBT measures lengthy duties—these taking longer than 50ms.

When a browser hundreds your web site, there may be primarily a single line queue of scripts ready to be executing.

Any enter from the person has to enter that very same queue.

When the browser can’t reply to person enter as a result of different duties are executing, the person perceives this as lag.

Basically, lengthy duties are like that individual at your favourite espresso store who takes far too lengthy to order a drink.

Like somebody ordering a 2% venti four-pump vanilla, five-pump mocha whole-fat froth, lengthy duties are a serious supply of unhealthy experiences.

A Technical SEO Guide to Lighthouse Performance MetricsWhat Causes a Excessive TBT on Your Web page?

Heavy JavaScript.


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That’s it.

Tips on how to See TBT Utilizing Chrome Devtools

A Technical SEO Guide to Lighthouse Performance MetricsTips on how to Repair Poor TBT

  • Break up Lengthy Duties.
  • Optimize your web page for interplay readiness.
  • Use an online employee.
  • Scale back JavaScript execution time.

Assets For Bettering TBT

First Contentful Paint (FCP)

What it represents: FCP marks the time at which the primary textual content or picture is painted (seen).

Lighthouse Efficiency rating weighting: 15%

What it measures: The time after I can see the web page I requested is responding. My thumb can cease hovering over the again button.


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The way it’s measured: Your FCP rating in Lighthouse is measured by evaluating your web page’s FCP to FCP instances for actual web site knowledge stored by the HTTP Archive. Your FCP will increase whether it is quicker than different pages within the HTTP Archive.

Is First Contentful Paint a Internet Core Very important? No

FCP Scoring

Purpose: Obtain FCP in < 2 seconds.

FCP time

(in seconds)

Coloration-coding FCP rating (HTTP Archive percentile)
0–2 Inexperienced (quick) 75-100
2-4 Orange (reasonable) 50-74
4 Crimson (gradual) 0-49

What Components Can Be A part of FCP?

The time it takes to render the primary seen ingredient to the DOM is the FCP. Something that occurs earlier than a component that renders non-white content material to the web page (excluding iframes) is counted towards FCP.

Since iframes usually are not thought of a part of FCP, if they’re the primary content material to render, FCP will proceed counting till the primary non-iframe content material hundreds, however the iframe load time isn’t counted towards the FCP.


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The documentation round FCP additionally calls out that’s usually impacted by font load time and there are tips for improving font loads.

FCP Utilizing Chrome Devtools

  1. Open the web page in Chrome.
  2. Navigate to the Efficiency panel of Dev Instruments (Command + Choice + I on Mac or Management + Shift + I on Home windows and Linux).
  3. Click on on the FCP marker within the Timings part.
  4. The abstract tab has a timestamp with the FCP in ms.

Tips on how to Enhance FCP

To ensure that content material to be exhibited to the person, the browser should first obtain, parse, and course of all exterior stylesheets it encounters earlier than it may well show or render any content material to a person’s display.

The quickest strategy to bypass the delay of exterior assets is to make use of in-line kinds for above-the-fold content material.

To maintain your web site sustainably scalable, use an automated tool like penthouse and Apache’s mod_pagespeed. These options will include some restrictions to functionalities, require testing, and will not be for everybody.

Universally, we are able to all enhance our web site’s time to First Contentful Paint by reducing the scope and complexity of style calculations.


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If a mode isn’t getting used, take away it. You may establish unused CSS with Chrome Dev Tool’s built-in Code Coverage functionality.

Use higher knowledge to make higher selections.

Much like TTI, you possibly can capture real user metrics for FCP using Google Analytics to correlate enhancements with KPIs.

Velocity Index

What it represents: How a lot is seen at a time throughout load.

Lighthouse Efficiency rating weighting: 15%

What it measures: The Velocity Index is the common time at which seen components of the web page are displayed.

The way it’s measured: Lighthouse’s Velocity Index measurement comes from a node module known as Speedline.

You’ll should ask the kindly wizards at webpagetest.org for the specifics however roughly, Speedline scores fluctuate by the dimensions of the viewport (learn as machine display) and has an algorithm for calculating the completeness of every body.

A Technical SEO Guide to Lighthouse Performance MetricsIs Velocity Index a Internet Core Very important? No

SI Scoring


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Purpose: obtain SI in < 4.3 seconds.

SI time

(in seconds)

Coloration-coding FCP rating (HTTP Archive percentile)
0–4.3 Inexperienced (quick) 75-100
4.4-5.8 Orange (reasonable) 50-74
5.8+ Crimson (gradual) 0-49

Tips on how to Enhance SI

Velocity rating displays your web site’s Crucial Rendering Path. A “crucial” useful resource implies that the useful resource is required for the primary paint or is essential to the web page’s core performance.

The longer and denser the trail, the slower your web site will probably be to supply a visible web page. In case your path is optimized, you’ll give customers content material quicker and rating larger on Velocity Index.

How the Critical Path Affects Rendering

A Technical SEO Guide to Lighthouse Performance Metrics

Lighthouse suggestions generally related to a gradual Crucial Rendering Path embrace:


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  • Reduce main-thread work.
  • Scale back JavaScript execution time.
  • Reduce Crucial Requests Depth.
  • Eradicate Render-Blocking Assets.
  • Defer offscreen photographs.

Time to Interactive

What it represents: Load responsiveness; figuring out the place a web page appears to be like responsive however isn’t but.

Lighthouse Efficiency rating weighting: 15%

What it measures: The time from when the web page begins loading to when its foremost assets have loaded and are in a position to answer person enter.

The way it’s measured: TTI measures how lengthy it takes a web page to grow to be absolutely interactive. A web page is taken into account absolutely interactive when:

  • The web page shows helpful content material, which is measured by the First Contentful Paint.
  • Occasion handlers are registered for many seen web page components.
  • The web page responds to person interactions inside 50 milliseconds.


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Is Time to Interactive a Internet Core Very important? No

TTI Scoring

Purpose: obtain TTI rating of lower than 3.8 seconds.

TTI Rating

(in seconds)

0–3.8 Inexperienced (quick)
3.8 – 7.3 Orange (reasonable)
7.3+ Crimson (poor)

Cumulative Format Shift (CLS)

What it represents: A person’s notion of a web page’s visible stability.

Lighthouse Efficiency rating weighting: 5%*

* Anticipate CLS to extend in weighting as they work the bugs out. Good wager says This autumn 2021.

What it measures: It quantifies shifting web page components via the tip of web page load.

The way it’s measured: Not like different metrics, CLS isn’t measured in time. As a substitute, it’s a calculated metric based mostly on the variety of frames during which components transfer and the whole distance in pixels the weather moved.

A Technical SEO Guide to Lighthouse Performance MetricsCLS Scoring

Purpose: obtain CLS rating of lower than 0.1.


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CLS Rating Coloration-coding
0–0.01 Inexperienced (good)
0.1-0.25 Orange (wants enchancment)
0.25+ Crimson (poor)

What Components Can Be A part of CLS?

Any visible ingredient that seems above the fold sooner or later within the load.

That’s proper – when you’re loading your footer first after which the hero content material of the web page, your CLS goes to harm.

Causes of Poor CLS

  • Pictures with out dimensions.
  • Advertisements, embeds, and iframes with out dimensions.
  • Dynamically injected content material.
  • Internet Fonts inflicting FOIT/FOUT.
  • Actions ready for a community response earlier than updating DOM.

Tips on how to Outline CLS Utilizing Chrome Devtools

  1. Open the web page in Chrome.
  2. Navigate to the Efficiency panel of Dev Instruments (Command + Choice + I on Mac or Management + Shift + I on Home windows and Linux).
  3. Hover and transfer from left to proper over the screenshots of the load (be sure that the screenshots checkbox is checked).
  4. Look ahead to components bouncing round after the primary paint to establish components inflicting CLS.

Tips on how to Enhance CLS

When you establish the ingredient(s) at fault, you’ll have to replace them to be secure through the web page load.


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For instance, if slow-loading adverts are inflicting the excessive CLS rating, you could need to use placeholder photographs of the identical dimension to fill that house because the advert hundreds to stop the web page shifting.

Some widespread methods to enhance CLS embrace:

  • All the time embrace width and top dimension attributes on photographs and video components.
  • Reserve house for advert slots (and don’t collapse it).
  • Keep away from inserting new content material above present content material.
  • Take care when putting non-sticky adverts close to the highest of the viewport.
  • Preload fonts.

CLS Assets


The complexity of efficiency metrics displays the challenges dealing with all websites.

We use efficiency metrics as a proxy for person expertise – meaning factoring in some unicorns.


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Instruments like Google’s Test My Site and What Does My Site Cost? will help you make the conversion and customer-focused arguments for why efficiency issues.

Hopefully, as soon as your undertaking has traction, these definitions will aid you translate Lighthouse’s single efficiency metric into motion tickets for a talented and collaborative engineering crew.

Observe your knowledge and shout it from the rooftops.

As a lot as Google struggles to quantify qualitative experiences, search engine optimization professionals and devs should decode tips on how to translate an idea into code.

Check, iterate, and share what you be taught! I sit up for seeing what you’re able to, you lovely unicorn.

Featured picture credit score: Paulo Bobita

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