When NOT to use Google Lighthouse

Summary: There are numerous tools available for measuring the speed of your website, making it difficult to choose. This article explains why you should not always rely on the most popular tool, Google Lighthouse.

When NOT to use Google Lighthouse

What does Google Lighthouse measure?

Google Lighthouse measures a variety of metrics, including performance, accessibility, SEO, PWA, and best practices. We concentrate primarily on the performance aspect. In a nutshell, Google Lighthouse measurement is a snapshot. A simulated "visitor" visits your website and assigns a score. You are evaluated based on five metrics:

  1. First Contentful Paint
  2. Largest Contentful Paint
  3. Total Blocking Time[*]
  4. Cumulative Layout Shift
  5. Speed Index[*]

[*] Good to know is that Google Lighthouse is a lab data test, because of this, 2 of the 5 metrics are also only observable in the lab.

Because it performs a lab data test, it does not measure the experience of your visitors. With real user monitoring tools like RUMvision, you gain insight into your so-called field (RUM) data.

Why Lighthouse and Core Web Vitals are different

When people talk about Core Web Vitals, we are talking about data from real users. Google Lighthouse is commonly referred to as the pagespeed score.

One significant aspect to understand is that Lighthouse doesn't actually scroll or interact with web pages. Additionally, there's an often overlooked detail where some third-party elements could be hidden behind a cookie notice. If Lighthouse hasn't accepted this notice, these elements remain inaccessible during its analysis.

Check your Interaction to Next Paint

Enter the URL of your website (for example, https://www.google.com) to get the aggregated Core Web Vitals data for the last six months for all pages or specific URL's if they have enough traffic.

FID and INP

When looking at metrics like FID and INP, similarities emerge. However, these similarities can become more pronounced discrepancies when there's a mistake, such as using a JavaScript delaying plugin. This can skew the results and give you a misleading representation of the site's performance.

FCP and LCP

TTFB might have gotten worse (for example, if the cache is invalidated, pages have to be re-rendered again), so public data might not show improvements (but more detailed RUM data might be able to show the difference).
Perhaps the site was already doing very well. So, while Lighthouse's conditions might show how FCP/LCP has improved, real users won't notice it in their own.

Lighthouse vs. Real User Data

Now that we know there is a difference in types of data and that Lighthouse is lab data we can show how different these scores are. In the screenshots below you can see on the left a Pagespeed Insights test (real data) and on the right a Lighthouse test of the same website.

This is one of the fastest-scoring websites in our tool, yet they only have a pagespeed score of 63. This does not represent perceived webperformanmce at all in this case, and continuing to focus on this score may be wasted time.

We fixate too much on the Lighthouse score

If you are not a developer, the Lighthouse score does not benefit you very much. This is because it does not represent the experience of your real users, but rather how clean the code is and how many recommendations the developer has followed. It is a tool for diagnosing a cold page load when certain pre-defined circumstances apply. One could even argue that using a single pagespeed score is a money-leaking approach.

Google Lightouse

The Lighthouse/page speed score are not the Core Web Vitals. Next time you see a high score, double-check that the actual visitors had a good experience with this Core Web Vitals checker.

What data you should focus on

As a stakeholder or non-developer, you need to start looking at the Core Web Vitals. This is based on your real users. With free tools like Pagespeed Insights, which you also see above, and our free UX score checker, you can get an indication of how well your visitors experience your website. However, this also lacks a lot of nuances.

  • 28 days delayed;
  • No idea what causes a bad score;
  • Only reports logged-in Google Chrome Users;
  • Understanding only a subset of your visitors (percentile);
  • No deep dive.

You've probably sensed it coming, but this is where RUMvision comes in. If you are serious about improving the speed of your website, you must have an RUM tool or you will always be one step behind.

Different levels of page speed tools

In terms of page speed, there are different levels and tools used for each level of knowledge. We often hear from clients what they used before RUMvision:

Level 1: Google Lighthouse

Level 2: Pagespeed Insights

Level 3:Core Web Vitals history

Level 4:RUMvision

This is also a very nice sequence because it transitions from lab data to real-world user data that lags and lacks many nuances, to a tool that can fully map everything in real-time. Like many merchants, do you want to get to the highest level of monitors with good guidance? We would like to help you get up to speed.

Just because Lighthouse is "level 1" does not mean it is a useless tool. It is very easy to start with, but at some point, as a stakeholder, you need to start looking beyond this score to increase this piece of maturity in your business.

Why you need a RUM tool

If you want to get started with page speed and give your visitors a good experience, we recommend using a RUM tool. That way you will always have instant insights into your performance. Are you making a change now that will have a good impact on the UX? Then you will see this depending on the number of visitors within minutes.

The goal is to give all visitors a good experience, simply because it makes more money. A Lighthouse score of 90+ isn't going to help you do that (unfortunately). The Core Web Vitals and our very own (F)UX score will.

UX score core web vitals

Easily track performance data as a stakeholder

We know better than anyone that Lighthouse is easy to understand and have taken inspiration from it. So on our audience dashboard, we also gave the metrics performance scores, just based on your real users. This way, monitoring performance based on level remains very attractive and you pay attention to data that is correct.

monitor core web vitals

So are you looking for a tool that shows the right data for you and makes monitoring, analysing and optimising Core Web Vitals easier for your whole team? Then RUMvision is ideal. If you're currently mainly working with Lighthouse, try to get a firm grip on the data from real users that you can also find on Pagespeed Insights.

Sparring on how mature your company is in this area? Then get in touch and we'll be happy to help think about the next step(s).

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