Google Lighthouse score

In order to measure loading times or analyse web performance, many of us turn to PageSpeed Insights. But have you ever wondered how the scores are calculated? What does it really mean to score 50, 70 or 90? And is it even possible to reach 100? Let’s take a look at how Google does its maths.

Lighthouse, a light in the darkness 

Having recently explored the workings of Test My Site, it’s time we looked more closely at PageSpeed Insights. 

Since 2018, the scores provided are those calculated by Lighthouse and are therefore synthetic to a certain degree. 

These are the web performance indicators collected by Lighthouse and factored into your score on PageSpeed Insights, listed by weighting (the rest is explained here):

It’s immediately apparent that TTI is weighted heavily, despite being a relatively recent indicator that has yet to be set in stone. As such, even though we are all seeking to improve our scores, this provides another reason to adopt a broader perspective.

As a reminder, this is what each metric represents: 

Web performance metrics

(Source: treo.sh)

When testing a page’s URL, PageSpeed Insights will first produce scores between 0 and 100 for mobile and desktop. A pair of tabs in the top left of the results page allow you to switch between them, displaying the mobile score by default.

So, what do the scores actually mean? It’s simple: a score from 1 to 49 is considered slow, 50 to 89 is considered average, and 90 to 100 is considered fast (if the score is 0, chances are Lighthouse encountered a bug).

The rating scale is calibrated according to measurements collected on the world’s largest sites by HTTP Archive. The highest score possible is 100, which represents the 98th percentile; a score of 50 represents the 75th percentile. In other words, a score of 50 still ranks your website among the top 25% in terms of performance. Nevertheless, the orange colour code can give the impression that this is an average or poor result.

Page Speed Insight score range

The highest scores are rarely attributed due to the scoring methodology employed, which always assumes the worst-case scenario. Specifically, the mobile score provided by default is often lower than the desktop score, because mobile web performance is harder to optimise. After all, there are greater network constraints on mobile, and a smartphone has less processing power than a desktop computer. In addition, Lighthouse actually simulates a 4G connection that is considerably slower than much of the network in France.

Testing the same page several times yields different scores. Why? 

Have you ever tested the same page multiple times with different results? As explained in Google’s Scoring Guide, your Lighthouse/PageSpeed Insights score may vary because conditions themselves may differ between tests: network quality, third-party scripts, ads, etc. Lighthouse is also limited insofar as it only takes one-off measurements, whereas genuine statistical data requires repeated measurements to account for potential aberrations. 

Harsh but fair

Getting an average or poor score does not mean that the site is unusable – far from it! To illustrate, take a look at the top 20 sites in the mobile web performance rankings for June 2019 (this is a ranking of the most visited French websites)– with home-page scores on PageSpeed Insights in the right-hand column.
Among the most-visited French mobile websites, these are the top 20 in terms of web performance, even though 70% have an average score, only 3 are above 90 and some are even below 50. As we have seen, these scores are variable from one test to the next. While those listed in the table below represent a snapshot at time T, they still provide a reasonable indication of web performance. If you would like to obtain a more reliable score, it’s best to take multiple tests and calculate the median of scores provided – a task that is easily automated:

Rankings Website Webperf Score Speed Index PSI Score
1 Service-Public.fr 1458 1788

72

2 Wikipedia 1580 1797

93

3 Ouest France 2136 2603

72

4 Bing 2189 2212

94

5 Le Monde 2231 2147

78

6 YouTube 2258 2352

57

7 RTL 2342 2708

75

8 PayPal 2342 2608

81

9 franceinfo 2350 2720

71

10 Google 2351 2763

96

11 Groupon 2522 2443

64

12 Twitter 2625 2527

57

13 PagesJaunes 2739 3192

70

14 Cdiscount 2756 2268

62

15 Météo France 2780 3009

56

16 Amazon 2882 2101

44

17 France Televisions 3135 3061

40

18 La Redoute 3555 3905

79

19 eBay 3681 3262

65

20 Booking.com 3761 2926

48

PageSpeed Insights uses a strict scoring system that encourages web performance optimisation – and that’s a good thing! Although you shouldn’t interpret these results to the letter (or indeed the number), they do invite reflection on how to improve loading times. Any score “in the red” signifies that it’s time to focus on optimising the performance of your website – and that’s exactly our core business

In part two, PageSpeed Insights: how to interpret and apply results 2/2, we shall discuss the more specific indicators on PageSpeed Insights, and what to make of the recommendations provided in each section of the results page.

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