Timeline of web vitals

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This is a timeline of web vitals, a set of metrics championed by Google that relate to how well web pages load. Of particular interest are three metrics Google calls core web vitals, whose values in field data are used to inform a "page experience" score that affects search ranking on mobile devices.

List of web vitals and corresponding aspects of performance

Note that for the three web vitals measured in both field and Lab data, the thresholds for good and poor for field data match the thresholds for good and poor on Lab data (measured using Lighthouse) respectively. This has been the case since June 2021 updates to field data thresholds and the corresponding release of Lighthouse v8. The desktop Lab data thresholds differ for some web vitals, and are not in the table below to keep it simple.

Web vital Aspect of performance Core web vital? Reported in field data (PageSpeed Insights, CrUX report)? Reported in Lab data (Lighthouse, PageSpeed Insights)? Reported in WebPageTest web vitals?[1][2] Threshold for "good" on field data (if applicable) and mobile Lab data (if applicable) (in milliseconds except CLS that is unitless) Threshold for "poor" on field data (if applicable) and mobile Lab data (if applicable) (in milliseconds except CLS that is unitless)
First Contentful Paint (FCP) Loading speed No Yes Yes Not online, but supported in code 1,800 3,000
Largest Contentful Paint (LCP) Loading speed Yes Yes Yes (since v6) Yes 2,500 4,000
Speed Index (SI) Loading speed No No Yes (some variant since v2) No 3,400 5,8000
Time to Interactive (TTI) Loading speed, interactivity/responsiveness No No Yes (since at least v5, some variant since v2) Not online, but supported in code 3,800 7,300
Total Blocking Time (TBT) Interactivity/responsiveness No No Yes (since v6) Yes 200 600
First Input Delay (FID) Interactivity/responsiveness Yes Yes No No 100 300
Cumulative Layout Shift (CLS) Visual stability Yes Yes Yes Yes 0.1 0.25

Numerical and visual data

The table below is based on monthly summaries provided in the CrUX announce mailing list.[3] Note that the total number of origins is not necessarily the denominator for the percentages; individual percentages have as denominator only the number of origins for which there was sufficient data for that particular metric.[4]

Year and month (YYYY-MM) Number of origins covered in dataset Percentage good Largest Contentful Paint (LCP) Percentage good First Input Delay (FID) Percentage good Cumulative Layout Shift (CLS) Percentage good on all core web vitals Special notes
2020-05 7,103,486 46 86 64 27 corrected data to restrict denominator[4]
2020-06 7,501,835 47.84 86.74 57.45 25.34
2020-07 7,527,878 47.6 86.9 57.2 25.2
2020-08 7,773,359 47.1 86.9 57.1 24.9 bump in origin coverage due to a data pipeline change that now accepts all user experiences with a FCP[5]
2020-09 7,937,088 46.8 87.6 55.8 24.6
2020-10 8,050,755 47.0 88.2 52.3 23.7
2020-11 7,942,408 47.0 88.8 51.1 23.5
2020-12 7,629,156 47.1 88.7 51.0 23.4
2021-01 8,185,540 47.81 89.28 50.25 23.71
2021-02 8,264,371 47.99 89.46 45.99 21.98
2021-03 8,326,310 49.01 89.66 51.84 24.81
2021-04 8,423,302 49.4 89.8 53.8 25.8
2021-05 8,411,670 50.7 89.9 60.4 29.2 Metric definition updates released in early June are applied to this data, including the 5-second session window for CLS, and at least partly explains the sharp increase in the percentage of origins with good CLS.[6]
2021-06 8,416,608 50.6 93.0 61.0 30.6 The effect of the double-tap-to-zoom FID metric change in Chrome 91, that improves measured FID, is largely seen in this month, and at least partly explains the sharp increase in the percentage of origins with good CLS.[7]
2021-07 8,174,923 50.0 93.2 61.2 30.4
2021-08 8,431,699 50.9 93.2 62.1 31.3
2021-09 8,660,068 50.9 93.3 65.4 32.8
2021-10 8,784,894 50.0 93.4 65.9 34.1 This is the first month when the percentages reported are based on the "optional FID" methodology adopted in PageSpeed Insights on June 10, 2021.[8][9]

Full timeline

Year Month and date (if available) Event type Type of tool, entity, or change Aspect of performance (specific metrics in parentheses) Type of measurement (field data (from real users; aka Real User Monitoring (RUM)) or Lab data (from a bot or synthetic local testing))? Event
2007 July New product Public diagnostic tool Loading speed Lab data Yahoo! launches YSlow, a Firefox add-on built on top of Firebug, to help debug speed issues on websites.[10][11] YSlow operationalilzes Yahoo! Chief Performance Officer Steve Souders' 13 rules for high-performance web sites (also covered in his book "High Performance Web Sites").[12] Development on it would cease after 2014 as people migrated to other tools like Google's Page Speed, Webpagetest, and later Lighthouse.[13][14]
2009 June 5 New product Public diagnostic tool Loading speed Lab data Google releases Page Speed, a Firefox add-on built on top of Firebug, to help debug speed issues on websites. Commentators consider this to be Google's equivaent to Yahoo's YSlow tool.[15]
2012 April 26 Documentation commit Web standard Detailed timing information Field data, Lab data The first commit is made to the repository with the specification for the Navigation Timing API. The API allows for access to information on the fetching of the root document and its loading in the browser.[16]
2013 April 9 Documentation commit Web standard Detailed timing information Field data, Lab data The first commit is made to the repository with the specification for the Resource Timing API. The API allows for access to information on the timing of download of various resources.[17]
2014 September 29 Documentation commit Web standard Loading speed (Speed Index) Field data The first commit is made to the repository with the specification for the RUM Speed Index (RUM stands for real user monitoring), an attempt to calculate a Speed Index based on field data. As of 2021, Speed Index is still calculated in the Lab.[18]
2014 November 18 Insight Metric definition, concept development Loading speed Field data, Lab data Tim Kadlec publishes his landmark post titled "Performance Budget Metrics" that classifies metrics used for performance budgets in four categories: (1) milestone timings (measuring when specific milestones in loading and rendering are achieved), (2) SpeedIndex (measuring how a page loads from start to finish), (3) quantity-based metrics (such as total number of requests, overall page weight, total image weight), and (4) rule-based metrics (such as PageSpeed and YSlow score).[19] Performance budgets would eventually be supported by build tools and diagnostic tools, and would be the subject of further analysis;[20] one analysis on web.dev would use a similar categorization of performance budget metrics (without a separate category for SpeedIndex).[21]
2016 January 15 Code commit Public diagnostic tool Loading speed, interactivity/responsiveness Lab data The first code commit to the Google Lighthouse GitHub repository is on this date.[22]
2016 June 30 Release Public diagnostic tool Loading speed, interactivity/responsiveness Lab data The Lighthouse v1.0.3 tag is created, suggesting the official release of Lighthouse 1.0.[23]
2016 September 14 Documentation comomit Web standard Loading speed (FCP) Field data, Lab data The first commit is made to the repository documenting the Paint Timing API. This API is used to calculate metrics such as the First Contentful Paint (FCP).[24] Initially supported only on Chromium browsers, the Paint Timing API would eventually be supported by Firefox (Gecko) and Safari (WebKit) as well.[25]
2017 February 1 Data collection Public dataset Loading speed Lab data Data for the HTTP Archive's Loading Speed dataset is available starting this day.[26]
2017 August 24 New version Public diagnostic tool Loading speed, interactivity/responsiveness Lab data Lighthouse v2.0.0 is released. The last commit for it is from May 19. At this point, the Performance section of Lighthouse has the following metrics: First meaningful paint, First interactive (beta), Consistenly interactive (beta), Perceptual Speed Index, and Estimated Input Latency.[27]
2017 October 1 Data collection Public dataset All Field data Data for the Chrome User Experience (CrUX) report, collected anonymously from Chrome users who have consented to the anonymous data collection, is available in a BigQuery dataset starting this day.[28]
2018 July 11 New version Public diagnostic tool Loading speed, interactivity/responsiveness Lab data Lighthouse v3.0.0 is released. The last commit for it is from June 28. This release includes changes to performance weights and scoring thresholds.[29]
2018 August 11 Release Public ranking tool Loading speed, interactivity/responsiveness Lab data The earliest Wayback Machine snapshot of webperf.xyz, a site that maintains a lleaderboard of sites based on their performance, is from this date. At launch time, it uses webpagetest.org for its data generation.[30] The tool would be referenced in an Atlantic article on August 23 about their efforts to speed up ad loading on their site.[31]
2019 January 16 New version Public diagnostic tool Loading speed, interactivity/responsiveness Lab data Lighthouse v4.0.0 is released. The last commmit for it is from January 15.[32]
2019 May 7 New version Public diagnostic tool Loading speed, interactivity/responsiveness Lab data Lighthouse v5.0.0 is released.[33] The metrics used in the performance score, with weights, are: First Contentful Paint (FCP) (23%), Speed Index (SI) (27%), First Meaningful Paint (FMP) (7%), Time To Interactive (TTI) (33%), First CPU Idle (FCI) (13%), and Max Potential FID (0%).[34]
2019 May 13 Documentation commit Web standard Visual stability (CLS) Field data, Lab data The first commit to the GitHub repository for the Layout Instability API is made on this day. This is a proposal by Google for adoption as a web standard, though as of June 2021 it has not been adopted and is only used on Blink-based browsers, which is effectively just Chromium browsers (mainly Chrome, Edge, and Opera).[35]
2019 May 15 Documentation commit Web standard Loading speed (LCP) Field data, Lab data The first commit to the GitHub repository for the Largest Contentful Paint API is made on this day. This is a proposal by Google for adoption as a web standard, though as of June 2021 it has not been adopted and is only used on Blink-based browsers, which is effectively just Chrome browsers (mainly Chrome, Edge, and Opera).[36]
2019 June 11 Announcement Metric introduction Visual stability (CLS) Field data, Lab data The blo post "Cumulative Layout Shift (CLS)" is published on web.dev. In the post, Google describes a new metric, Cumulative Layout Shift (CLS), that measures the totality of unexpected shifts during the lifecycle of a page. This builds upon the Layout Instability API that Google started workingg on recently.[37]
2019 August 8 Announcement Metric introduction Loading speed (LCP) Field data, Lab data The blog post "Largest Contentful Paint (LCP)" is published on web.dev. In the post, Google describes a new metric, Largest Contentful Paint (LCP), that measures how long it takes for the page's main content to have loaded.[38]
2019 September 19 Insight Public diagnostic tool All Lab data The page "Lighthouse performance scoring" gives insight into how Google Lighthouse scores are calculated. In particular, it explains that the log-normal distribution used for translating each of the web vitals into a score component is based on real website performance data on HTTP Archive.[39]
2020 April 30 Announcement Metric selection, thresholds All Field data, Lab data Google announces the selection of three of its metrics as core web vitals: Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS). The post also states that the 75th percentile of fielld data should be used for any assessment using web vitals, and specifies thresholds for good and poor values for each of the core web vitals.[40]
2020 May 5 Insight All Field data, Lab data Google publishes three blog posts, one for each of the three core web vitals (LCP, FID, and CLS), on strategies to optimize that core web vital.[41][42][43]
2020 May 12 New version Public diagnostic tool All Lab data In version 21.07, WebPageTest adds support for web vitals. WebPageTest, available at webpagetest.org, is a tool that can be used to report performance data on any web page.[44]
2020 May 19 New version Public diagnostic tool All Lab data Lighthouse v6.0.0 is released (it is expected to ship in the DevTools of Chrome 84, that releases July 14).[45] The performance scoring is updated, with three new metrics added: Largest Contentful Paint (LCP), Total Blocking Time (TBT), and Cumulative Layout Shift (CLS); two of which are among Google's three core web vitals. Three metrics from Lighthouse v5 are removed: First Meaningful Paint, First CPU Idle, and Max Potential FID. The weights are as follows: First Contentful Paint (FCP) (15%), Speed Index (SI) (15%), Largest Contentful Paint (LCP) (25%), Time To Interactive (TTI) (15%), Total Blocking Time (TBT) (25%), and Cumulative Layout Shift (CLS) (5%). This is the first time that Lab data includes a metric for visual stability (the specific metric being CLS).[34]
2020 May 21 Insight Thresholds All Field data In a lengthy blog post, Google describes how it decided on the 75th percentile of field data as the point to use for assessing each of its metrics, and also explains how thresholds were selected for each of the threee core web vitals: LCP, FID, and CLS. The decision for each metric is made by a combination of acceeptable user experience and achievability by webpages with current technology.[46]
2020 May 27 Release Private diagnostic tool All Field data Google updates Google Search Console adding a section on core web vitals. This section reports on field data performance on both mobile and desktop on each of the three core web vitals (CLS, LCP, and FID). It is based on the CrUX report. Rather than making all the data available for each url, Search Console groups urls together and reports on aggregate performance of the url groups.[47]
2020 May 28 Announcement Search algorithm update All Field data Google announces that at some point in the future, it will start using "page experience" as a ranking factor in search. The page experience signal combines core web vitals, mobile usability, security issues, whether the site is HTTPS, and no intrusive intersitials.[48][49]
2020 June 24 Implementation Web standard Loading speed (FCP) N/A A blog post on the Wikimedia Foundation's tech blog describes work donee by Wikimedia Foundation engineers to implement the Paint Timing API in WebKit, the engine used by Safari, thus making it available in future versions of Safari.[50]
2020 November 10 Announcement Search algorithm update All Field data Google announces that it will start using "page experience" as a ranking factor for mobile searches starting May 2021. The page experience signal combines core web vitals, mobile usability, security issues, whether the site is HTTPS, and no intrusive interstitials.[51][52]
2020 November 16 New version Public diagnostic tool, private diagnostic tool All Lab data GTmetrix, a performance optimization tool, switches from its legacy Timings and PageSpeed/YSlow measurements to using Lighthouse as its underlying engine; the new data includes Performance and Structure tabs, an overall GTMetrix grade, and a web vitals section that includes LCP, CLS, and Total Blocking Time (TBT).[53][54][55]
2020 December 17 New version Public diagnostic tool All Lab data Lighthouse v7.0.0 is released. It is expected to be shipped with Chrome 89 (released March 2, 2021) and becoms part of PageSpeed Insights on February 19, 2021.[56]
2021 February 17 Release Threshold update All Field data Field data available in Google Search Console and PageSpeed Insights are updated to consider a metric value at a threshold as being on the good side of the threshold. For instance, for CLS, the the threshold for good CLS is 0.1; a 75th percentile CLS value of exactly 0.1 would now be considered good.[57]
2021 April 7 Announcement Metric definition update Visual stability (CLS) Field data, Lab data Google announces a planned update to the calculation of Cumulative Layout Shift (CLS) to be fairer to long-lived pages. The update changes the definition of CLS to look at the largest CLS over 5-second windows with a 1-second gap. The rollout of this definition update would happen in June.[58][59]
2021 April 19 Release Private diagnostic tool All Field data A new section called "Page experience on mobile" is added to Google Search Console (where people can access this data for their own sites only). This is restriced to mobile searches; it reports on the percentage of urls and number of search impressions that have "good page experience" i.e., that perform well on core web vitals, mobile usability, security issues, whether the site is HTTPS, and ads experience.[60]
2021 April Insight Data All Lab data Searchmetrics publishes a study of core web vitals, looking at Lab data for over 2 million URLs on two of the three core web vitals (LCP and CLS) and a proxy for the third core web vital (using Total Blocking Time (TBT) instead of FID). YouTube is highlighted as an outlier that skews measurement.[61]
2021 April 19 Announcement Search algorithm update All Field data Google announces an updated timeline of mid-June to August for its rollout of page experience as a ranking factor for mobile searches. The previous announced rollout time was May 2021; the time extension is allegedly to give websites more time to prepare.[62]
2021 April 25 New version Public ranking tool All Lab data The backend for webperf.xyz is switched from webpagetest to Lighthouse on this date (or least, this is the earliest date with data recorded using Lighthouse).[63]
2021 May 4 Insight Cross-browser comparison of metric definition Loading speed (FCP) N/A A blog post on the WebPageTest blog describes challenges with comparing First Contentful Paint (FCP) across browsers. FCP is one of the first web vitals to be available across all major browsers with the release of Safari 14.1.[25]
2021 May 25 New version Browser behavior Interactivity/responsiveness (FID) Field data Chrome 91 disables double-tap-to-zoom when the viewport meta tag specifies width=device-width or initial-scale>=1.0, even when implicitly doing so, like for example in minimum-scale=1.5, maximum-scale=2. Because DTZ negatively impacts FID and the amount of pages where DTZ is disabled is increased, Google expects some sites to see FID improvements.[64]
2021 June 1  – 2, some changes a little later in June Release Metric definition update Visual stability (CLS) Field data Google updates its field data tools including the Chrome User Experience (CrUX) report, PageSpeed Insights, and Google Search Console, to use an updated definition of Cumulative Layout Shift (CLS) that looks at the largest CLS over 5-second windows with a 1-second gap. The planned change itself had been announced back in April, but it had not yet been reflected in the tools Google used to report on field data, as well as in Google's Lighthouse tool available on PageSpeed Insights and in Chrome Developer Tools (canary channel only, so not part of officially released Chrome yet).[65][66][67]
2021 early June Release Metric definition update Loading speed (LCP) Field data Some changes are made to LCP calculations to account for offscreen images and multiple images of the same size.[68]
2021 June 2 New version Public diagnostic tool All Lab data Lighthouse v8.0.0 is released. The release is made available on PageSpeed Insights immediately, and is expected to ship as part of Chrome 93. This includes Lab-side changes corresponding to field data updates for CLS released at around the same time; it also includes threshold changes for TBT and FCP and a reweighting: FCP: 15% to 10%, SI: 15% to 10%, TTI: 15% to 10%, TBT: 25% to 30%, and CLS: 5% to 15%. A new diagnostic aide called the Lighthouse Treemap is also released.[69][70]
2021 June 10 New version Public diagnostic tool All, FID Field data PageSpeed Insights field data now reports field data even for pages where some (but not all) of the field data metrics have insufficient data; in such cases, only the metrics with sufficient data are reported.[71] Rick Viscomi further clarifies on Twitter that this will most affect pages missing FID, since FID data tends to be much more likely to be missing than LCP or CLS data. Further, an origin with insufficient FID data will be considered to pass overall in core web vitals if it passes LCP and CLS; however, an origin must have LCP and CLS data in order to be considered passing on core web vitals.[9]
2021 June 15  – September 2 Release Search algorithm update All Field data The rollout of Google's "page experience on mobile" update happens during this period. The update is limited to mobile searches, and gives weight to a "page experience" factor that includes performance on core web vitals, mobile usability, security issues, whether the site is HTTPS, and ads experience.[72][73][74]
2021 June 24 Release Public diagnostic tool All Field data The HTTP Archive's Core Web Vitals Technology Report is released. This builds on the Chrome User Experience Report (CrUX report) as well as the HTTP Archive's data on CMS and technology stacks used by various websites, to provide a big-picture view of performance on core web vitals by CMS and technology stack.[75]
2021 August 31 New version Browser behavior Visual stability (CLS) Field data Chrome 93 makes changes to its calculation of cumulative layout shift (CLS) to ignore changes to offscreen elements countered with scroll anchoring, as well as layout shifts while dragging or resizing elements with a mouse. This is expected to improve the performance of websites on CLS.[76]
2021 September 21 New version Browser behavior Loading speed (FCP) Field data, Lab data Chromium is updated so that paint timing is not triggered by content that is not visible due to being under the effect of an opacity: 0 style. This change is part of the Chrome 94 release on September 21, 2021, and is also expected to affect other Chromium-based browsers such as Edge and Opera. The change is expected to increase (worsen) measured FCP (in field and Lab data) for some sites, without directly changing the end user experience.[77]
2021 November 3 New version Public diagnostic tool All Field data, Lab data A new version of the PageSpeed Insights UI is released.[78] Around this time (by November 15), PageSpeed Insights is moved to pagespeed.web.dev.[79]
2021 November 13, November 15 New version Public diagnostic tool All Lab data Starting November 13, PageSpeed Insights starts using Lighthouse 9.0.0.[79] On November 15, Lighthouse 9.0.0 is officially released. It is expected to be part of stable Chrome starting Chrome 98.[80][81]
2022 February  – March Release Search algorithm update All Field data Per an announcement on November 4, 2021, Google plans to start the rollout of using page experience as a ranking factor on desktop during this period. Page experience on desktop includes all the core web vitals as well as all the other page experience signals on mobile except mobile-friendliness.[82][83]

See also

References

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