Sharing great detail from @eva2000 first posted in the performance category. Links to tutorials included in this post.
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Heads up, according Chrome Dev Summit talks/slides looks like in January 2020, Google Lighthouse v6 will be released so Google PageSpeed Insights v6 will be out too. They are adjusting the weighting and adding new metrics for the Pagespeed score - moving First Contentful Paint (FCP) to a higher weighting than it has in Google PageSpeed v5 and also adding a Total Blocking Time (TBT) and Largest Contentful Paint (LCF) metric https://twitter.com/bunopus/status/1193999246812278784. Both First CPU Idle and First Meaningful Paint (FMP) metrics are being removed.
This also makes sense with Google Webmaster Console adding a new Speed Report which also evaluates and displays page speed based on First Contentful Paint (FCP) and First Input Delay (FID) Official Google Webmaster Central Blog: Get faster with the new Speed report in Search Console
Actual Google Dev Summit 2019 Youtube session that talks about this is here.
According to data compiled by Web Almanac 2019 By HTTP Archive, First Contentful Paint (FCP) metric is highly influenced by the type of device (desktop/mobile), ISP connection speed and geography.
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Google Lighthouse v6 is out now !
Webpagetest.org includes Google Lighthouse tests if you choose to enable them as outlined here. Which is same as Google PageSpeed Insights.
Note Google Lighthouse v6’s scoring metrics have changed with 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
Lighthouse 5 Audit Weight
- First Contentful Paint 20%
- Speed Index 27%
- First Meaningful Paint 7%
- Time to Interactive 33%
- First CPU Idle
Lighthouse 6 Audit Weight
- First Contentful Paint 15%
- Speed Index 15%
- Largest Contentful Paint 25%
- Time to Interactive 15%
- Total Blocking Time 25%
- Cumulative Layout Shift 5%
As you can see previously Time To Interactive was weighted at 33% of the total score in Lighthouse v5 so even if you had alot of Javascript slowing Time To Interactive, if you had a good First Contentful Paint (20%) and Speed Index (27%) which totaled 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 totaled 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.
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Google Search Console’s Speed Report has now switched to using Core Vital Metrics as well and tracks both Largest Contentful Paint (LCP) and First Input Delay (FID) for real world visitors to your web site via Google Chrome User Experience Report (CRuX) data. You can measure your lab and field/origin metrics via Google PageSpeed Insights v6 & Google Lighthouse v6 metrics.
For my forums, majority of the threads are picking up the title bar’s H1 tag as the LCP item. So thought I’d use Element Timings API to measure when my title bar’s H1 tag actually renders for me in the real world. FYI, Google Search Console Vital metrics reported my forum index having a LCP of 1.5s for 75% of visits which would of been made up of both guests + logged in members.
For forum index page measured H1 title bar rendered times for me in Brisbane, Australia hitting my forums in US West Coast behind Cloudflare
- Logged in member/visitor no CF full HTML page cache between 2000-2500ms so ~2-2.5 seconds
- Logged in member/visitor no CF full HTML page cache but watching a video on my computer blew out the H1 tag render time to 19790ms ! Yes 19.79 seconds !
- Guest logged out visitor with CF full HTML page cache between 900-1300ms with random variation spikes of 2400-4900ms (could be factors local to my PC/apps in use)
This goes to show in real world, your visitors maybe also multi-tasking on their computers which may impact their browsing page load experience and thus impact your real world Google Search Console Speed LCP/FID metrics and Google Page Speed Insight’s reported field/origin metrics 
Below is Opera browser dev tools console reported Elements Timing API render times for my forum index’s title bar H1 tag.
- left = logged in member = 2096ms render time
- middle = logged in member while watching a video at same time = 19790ms render time
- right = guest logged out visitor = 1032ms render time
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FYI, for Webpagetest TTFB is the TTFB of the Index request specifically and NOT Webpagetest reported summary First Byte which is a different metric https://community.centminmod.com/threads/cloudflare-automatic-platform-optimization-for-wordpress-cache-effectiveness.20494/
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.
First Byte summary not same as Index request TTFB
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