Why the Lighthouse mobile score of my website is bad?

Clients sometimes message us and say that while their desktop website score is above 90, their mobile score is lagging. In this article, we explain why that is.

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Tl;dr: If your mobile score is low, don't panic. It doesn't mean that your UX and SEO will suffer. Most websites out there have a poor Lighthouse mobile score, even if their Desktop scores are amazing.

How are Lighthouse scores calculated?

What is Lighthouse?

Lighthouse is a tool for improving the quality of your web pages. It audits different aspects of your website - performance, SEO, accessibility and PWA and gives you a score out of 100.

What is Pagespeed?

Pagespeed is a tool created by Google, which uses the scores created by Lighthouse and adds additional advice on how to improve these scores.

Lighthouse score calculations

Lighthouse uses several different metrics to calculate the performance score of your website and assigns them different weighting for each one:

For example, Total Blocking Time and Largest Contentful Paint are much more important metrics than First Contentful Paint, but often enough, these metrics are related to improving one will affect the other as well.

Why do my Lighthouse scores fluctuate so much?

As you might have already noticed, the scores of the test change every time you run them - sometimes with a 10-20 point swing!

This variability has been adknowledged by the Lighthouse dev team. Here are the common factors that affect the scoring between tests:

  • Internet speed
  • Chrome extensions
  • Ads on the page
  • Hardware that the test is run on

If you are interested to read more about it, check out the analysis paper created by Google.

Network Throttling in Mobile tests

According to Lighthouse's official documentation, they simulate the bottom 15% of mobile users or the bottom 25% of 4G connections and the top 25% of 3G connections.

This means that even if your website loads amazingly well on your mobile phone and the mobile phone of your users, you might still get a really bad Lighthouse mobile score.

Investigating popular websites and their Lighthouse mobile scores

The following websites and businesses take pride in their web performance and speed and have an amazing score on Desktop (often >90). Let's see how their mobile scores compare.

All tests are conducted on the Pagespeed website on the latest Chrome browser in incognito using a 2021 Macbook Pro. I'll conduct each test 3 times, then take an average to remove any fluctuation from the Lighthouse scoring.

These tests were conducted on 13th March 2022

Test #First Contentful Paint (s)Time to Interactive (s)Speed Index (s)Total Blocking Time (ms)Largest Contentful Paint (s)Cumulative Layout Shift (s)Mobile Score
Test 127.33.311502.4067
Test 228.3417002.1063
Test 328.34.313902.1064

Test #First Contentful Paint (s)Time to Interactive (s)Speed Index (s)Total Blocking Time (ms)Largest Contentful Paint (s)Cumulative Layout Shift (s)Mobile Score
Test 177.1708.10.03254
Test 37.487.409052

Test #First Contentful Paint (s)Time to Interactive (s)Speed Index (s)Total Blocking Time (ms)Largest Contentful Paint (s)Cumulative Layout Shift (s)Mobile Score
Test 22.89.649202.90.00260

Test #First Contentful Paint (s)Time to Interactive (s)Speed Index (s)Total Blocking Time (ms)Largest Contentful Paint (s)Cumulative Layout Shift (s)Mobile Score
Test 12.913.45.311706.50.25828
Test 22.913.75.414605.20.25829
Test 32.913.45.29905.20.56528

As you can see, even companies that pride themselves on their great marketing, SEO and website speeds and have built businesses around this can output a mobile score in the mid-60s (lower in the case of Hubspot and Apple).

Furthermore, remember that these companies have the best web developers and budgets out there. Chances are that your company has fewer resources at hand, so my humble opinion is that if you have a mobile score in the range of 40/50, you should be proud of that! Also note that if I had picked other businesses in other areas (eg e-commerce), I'm sure that the results will be a lot lower.

Pro tip: conduct a Web Vitals score research like the one I've done above for your competitors and see how their desktop/mobile scores compare to yours.

Will improving the mobile score improve the SEO of my website?

Not necessarily. If your website is already scoring >90 on desktop and >40 on mobile, it's safe to say that your business has done as well as anybody else. It's much better to spend more time improving other aspects of your technical SEO, writing content and building backlinks than to try to bump your average mobile score by a couple of points which will bring unknown benefits.

Don't get addicted to Lighthouse scoring

Lighthouse is a really useful tool, but instead of focusing on the scores, it's much better to instead analyze what recommendations they give you below the scores and investigate if there are good ways to improve the loading speed of your website.

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