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WebPerf PageSpeed Google Page Speed Insights and Google Core Web Vital metrics

Discussion in 'Web Development & Web Performance' started by eva2000, Nov 26, 2020.

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  1. eva2000

    eva2000 Administrator Staff Member

    May 24, 2014
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    Google PageSpeed Insights uses Google Lighthouse v6 API to test Google's page experience signal new Core Web Vitals metrics for both test lab and real world field data test (via Chome User Experience Report - CRuX) of your site’s theme/layout as well and how it’s put together. Your decisions on what theme, layout and number of 3rd party requests/ads you place on your sites' pages and the order of loading those assets will impact the page speed metrics measured.

    You can read the updated HTTP Archive Web Almanac 2020's Google Lighthouse Performance section for more details.

    The gist of Google's user page experience signal measurements is that if you have a lot of javascript loading on your page, it will impact a users page loading experience and thus you'd lower your scores. The less javascript you have on a page, the better the user page experience.

    Google PageSpeed Insights

    Note for purpose of Google user page experience signals, Google is looking at real world Core Web Vital metrics in the form of field data and not the synthetic lab data which Google Page Speed Insight's single score is based on. So do not read too much into the single score reported for lab data. If there isn't enough real world field data for the page tests, Google Page Speed Insights will report the origin real world field data.

    Example for this forums when not enough real world field data, it will report the Origin Summary data collected over the last 28 days and assess whether you pass the Core Web Vital assessment which is derived from the same data in your Google Webmaster Tool's Core Web Vital metrics tracking. This also means if you're optimizing your pages and page speed, you'd need to wait at least 28 days to see the full picture for real world field measurements. You can use the lab metric data as a relative guide in tracking your improvements and changes in conjunction with real world field metrics. It's the reason why Google PageSpeed Insights measures and presents both lab and field metrics for you to see.


    Google Webmaster Tools Core Web Vital Metrics

    Real world field data via Google Webmaster Tool's Core Web Vital metrics which is sourced from Chome User Experience Report (CRuX) data.





    Google Lighthouse v6

    Google Lighthouse v6’s scoring metrics have changed what they test and their weightings with the removal of First CPU Idle and First Meaningful Paint and addition of Total Blocking Time, Largest Contentful Paint and Cumulative Layout Shift and also changes to the score weighting to further emphasize the impact of excessive Javascript. See Lighthouse performance scoring 2

    Lighthouse v5 Audit Weight

    Previous Google Lighthouse v5 weighting and metrics

    Lighthouse v6 Audit Weight

    Current Google Lighthouse v6 weighting and metrics
    As you can see previously Time To Interactive was weighted at 33% of the total score in Lighthouse v5 so even if you had a lot of Javascript slowing Time To Interactive, if you had a good First Contentful Paint (20%) and Speed Index (27%) which totalled 47% of the score, you could still get a relatively high Lighthouse score.

    However, with Lighthouse v6 the weighting has changed. They have lowered the weighting on First Contentful Paint (15%) and Speed Index (15%) which totalled 30% of the score. And increased weighting on metrics that Javascript impact such as Time To Interactive (15%) and Total Blocking Time (25%) and Cumulative Layout Shift (5%) which have a combined 45% weighting to the overall score.

    This means if your web site is Javascript heavy, you’d expect lower Google Lighthouse v6 scores in Webpagetest and Google PageSpeed Insights v6.

    Page Speed Testing Tools

    For page speed tests, there are various tools online. Some are more useful and accurate for what they test. Some may use Google Lighthouse v6 engine but they use different servers or testing parameters so may not all show the same results i.e. whether cpu throttle on mobile tests is used or the network speed throttling is used and at what speed i.e. 3G, 4G, 5Mbp cable etc all my vary. Also, the tested web browser viewport size may differ which impacts how much of above fold viewport is rendered in the web browser and thus how many assets are rendered above the fold and below fold etc.

    The page speed testing tools which are my favourite to use include:
    1. for pagespeed testing - my guide at WebPerf - PageSpeed - How to use for page load speed testing shows you how to use WPT and also enable Google Lighthouse v6 optional tests too. WPT documentation. Note that the Webpagetest First Byte summary table isn't the same as a HTML documents actual request TTFB. The First Byte summary is a the total for Discovered + DNS + Connect + SSL + TTFB times. The First Byte is the time taken from navigationStart to the browser receiving the very first byte of data from the server. See Matt Hobb's explanation here.
    2. Google PageSpeed Insights - uses Google Lighthouse v6 API to test both lab and real world field Core Web Vitals metrics which Google will start using as part of their User Page Experience signals starting in May 2021. Google Webmaster Tool's Core Web Vital metrics tracking is the main way to track your real world user Core Web Vital metrics. Example for real world Google Core Web Vital metrics tracking via Chome User Experience Report (CRuX) which is released and updated every month here.
    3. New version of GTMetrix which has now switched to using Google Lighthouse v6 engine too Welcome to the new GTmetrix – powered by Lighthouse and Everything you need to know about the new GTmetrix Report (powered by Lighthouse)
    4. also uses Google Lighthouse v6 engine
    5. created by Wordfence Wordpress folks and tests using Google Lighthouse across several geographically based servers. Their help page.
    Last edited: Nov 26, 2020
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