I’d start with reading what thinkwithgoogle is testing at https://www.thinkwithgoogle.com/feature/testmysite/faq/ which is your real world user pagespeed from CRuX Chrome User reports from google analytics etc.
What is site speed?
Site speed measures the speed of all the pages that make up your site based on real world data collected via the Chrome User Experience Report (CrUX). The CrUX report is updated monthly and is publicly available. This public report is the source of current and historical data. The data is shown by country and effective connection type. Site speed is based on the moment your website begins to appear in a browser, which is known as 90th percentile First Contentful Paint (FCP). The site speed data is updated monthly and currently reflects data captured during October 2019.
And PageSpeed Insights
What is page speed?
Page speed is the speed of an individual page on your site. The page speed metric shows First Contentful Paint (FCP) as simulated in a lab environment and available from Google PageSpeed Insights (PSI). FCP tracks the moment at which your website begins to appear in the browser, providing feedback to the user that your page is loading. The points at which your page is fully visible and/or the the page is interactive often comes some time after this. The page speed metric shown is for 4G and is not specific to any given country. Each test may return different page speed results.
Is it normal that my page speed and site speed are different?
Yes, page speed and site speed will likely return different numbers. This is because site speed looks at the speed of all the pages on your site, while page speed just looks at the speed of an single page. As mentioned above, site speed is determined from field data collected over the past month, whereas page speed is derived from a lab metric collected on demand.
And for PageSpeed Performance Tutorials - Google PageSpeed & Webpagetest.org
Gist is PageSpeed is calculated on a mix of CRuX Chrome User Report and lab tests from PageSpeed Insights which tests 3G on slow mobile devices.
Sure CF can help with reduce size of images on mobile i.e. Mirage so that 1200KB sized page may end up at 900KB after image optimisations. But still 900/204 = 4.41 seconds.
In theory you’d need for a page size + ads + external scripts to not total more than 400KB for it to hit reasonable metrics for 3G mobile speed test.
Also these page speed testing tools like PageSpeed Insights aren’t perfect as they do’t fact in common page speed optimisations when tested like
- Browser level cache - repeat visits will have assets cached in browser so they don’t need to be transferred again so that 1200KB page might only end up having <400KB transferred over the network in real life while rest was in visitor’s browser cache.
CRuX Chrome User Report on other hand is from real world user data so may take into account browser cache.
You can also use Cloudflare Browser Insights https://blog.cloudflare.com/introducing-browser-insights/ and Google Analytics pagespeed data to drill into your CF accelerated web site’s analytic and page speed metrics and see which geographic region/pages are slow for you as well and which areas pull down your averages, median and percentile values for page speed.