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I tried to improve my Google Page Speed Insights mobile score by optimizing images on the website. I analyzed the website a few times - when I didn't do anything and when I replaced some not optimized images what was supposed to make things better. Each time I got a different score. First it was 49 or even 34 (when I didn't do anything) and then when I changed some images I got 54, 52, 49 or 34. It doesn't make much sense. Why does this happen and what can be done to improve the score?

Ania
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  • There can be multiple causes: server load, network variability, race conditions in your JavaScript causing the load order to change etc etc. If your site is inconsistent in loading you need to run a profile in the `performance` tab in developer tools a few times with network throttling applied to see what happens. At the moment that is the best information we can give as there isn't enough information in your question to help you diagnose. – GrahamTheDev Sep 24 '20 at 22:53

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In Page Insight there are Couple of Section "Score", "Field Data", "Origin Data", "Lab Data". The total score is based on all the four. If you optimized the code and run against page insight don;t look at score only see that was before and after the change in section "Lab Data". If your score improved in lab data you could see the total score go up after 28 days. In any case you don;t have to wait for 28 days instead click on the link "See Calculator" in lab data that will tell how your score look after 28 days.

From my experience tool is consistent the variation you are seeing are due to field data getting refreshed for your domain.

Thanks,

Sham
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  • "Score" is calculated directly from "Lab Data". The "origin data" and "field data" are real world data to let you see how the site performs "in the wild". The CRUX data (that you see in "origin data" and "field data") has no bearing on your score when you run a Page Speed Insights test. Read this about [Page Speed Insights and how scoring works](https://developers.google.com/speed/docs/insights/v5/about#score). Also [how scores are calculated](https://stackoverflow.com/a/62063118/2702894) might be useful so you know what to look for. – GrahamTheDev Sep 26 '20 at 07:40
  • I believe on recent change score is not calculated from lab data alone which was true earlier and not now from my experience. Also when you click on calculator you could see the same because calculator use lab data to show score. – Sham Sep 27 '20 at 03:43
  • I pointed you to the Google document that states clearly "This score is determined by running Lighthouse to collect and analyze **lab data** about the page." No mention of the other data. [Lighthouse is open source](https://github.com/GoogleChrome/lighthouse), so you can look through every line of code and learn how it works. Your experience has led you to incorrect conclusions I am afraid. With regards to the [scoring calculator](https://googlechrome.github.io/lighthouse/scorecalc/) it just shows you differences between v5 and v6. **lab data** is **simulated data each run** for clarity. – GrahamTheDev Sep 27 '20 at 08:20
  • [This is the documentation on variability](https://github.com/GoogleChrome/lighthouse/blob/master/docs/variability.md) that shows probable causes. That may be useful for you to understand what causes score differences. [You can even check the analysis that created that page](https://docs.google.com/document/d/1BqtL-nG53rxWOI5RO0pItSRPowZVnYJ_gBEQCJ5EeUE/edit). It may be useful so you understand where variance comes from. I hope all of that is useful but as I said, only "lab data" is used for scoring so if you are seeing variance those articles will hopefully explain why – GrahamTheDev Sep 27 '20 at 08:25