What is PageSpeed Insights by Google?

One of the most frequent questions our Customer Support team receives here at WP Rocket is “How can I get a 100/100 score with PageSpeed Insights?”.

How many times, while measuring your website speed, you asked yourself a similar question?

There’s no doubt that PageSpeed Insights by Google is one of the most internationally used page speed measurement tools, and for good reasons.

The Google team has always been on the front line of web performance optimization and tooling. Over the last ten years, they developed a series of performance tools dedicated to different types of users: from developers to marketers to amateur website owners, there’s a Google tool suitable for every need!

At the moment, we can count seven performance tools developed by Google:

Google’s paternity allowed PageSpeed to rapidly gain a high reputation since its launch in 2013.

However, as we already mentioned in our guide about page speed optimization for WordPress, the perfect speed test tool doesn’t exist: so PageSpeed Insights shouldn’t be considered the only voice of truth, but a good ally for your web page performance assessments, along with other equally valid tools.

With this guide, you’ll get to know PageSpeed Insights from its core; you’ll understand how its famous score system work and what does it exactly measure.

What is PageSpeed Insights by Google?

PageSpeed Insights (PSI) measures the performance of a page on both mobile and desktop devices, and then provides suggestions to improve that page.

PageSpeed Insights is powered by Lighthouse since the end of 2018. This means that the results in your PSI reports are based on the Lighthouse API.

When scanning a page, PageSpeed provides two types of data about it: Lab and Field data.

Lab data is collected in a controlled environment, that is with a set of predefined devices and network settings. It is effective for debugging performance issues, and its testing can be easily reproduced.
However, it might fail in catching real-world bottlenecks.

On the other hand, field data (also called Real User Monitoring or RUM) includes performance data coming from real page loads. It’s effective to capture true, real-world user experience, but the set of metrics is limited as well as its debugging potential.

What Kind of Information Does PageSpeed Provide About Your Site?

When you ask PageSpeed Insights to analyze a page, it will return different sections and indicators about the performance of that page.

In order of appearance:

Speed score:

PageSpeed Insights score for WP Rocket is 98

The speed score is based on Lighthouse lab data. We’ll see in the next chapter how it is calculated.

Field data:

Field data is based on real-world Chrome users experience over the last 30 days: it includes First Contentful Paint (FCP) and First Input Delay (FID).

Lab data:

Lab data tab on PageSpeed Insights

Lab data is based on a Lighthouse analysis on an emulated mobile device and mobile network. See the next chapter for the detail of the metrics included in this section.

Opportunities:

Opportunities tab by PageSpeed Insights

The Opportunities section includes recommendations about performance metrics that could improve page loading time. Each recommendation includes an estimation of the load time the page could save if the suggestions were implemented.

Diagnostics:

Diagnostics tabs PageSpeed Insights

The Diagnostics section provides recommendations about which best practices for web development should be added to the page.

Passed audits:

The Passed Audits section includes all the performance checks the page already meets and that don’t need any intervention.

In our opinion, three main reasons explain the popularity of PageSpeed Insights:

How Is PageSpeed Insight Score Calculated?

As we said, PageSpeed results are now powered by Lighthouse API.

The score makes no exception: the speed rating that you see on the top of your PageSpeed report is based on the lab data analyzed by Lighthouse.

The most important thing to notice in this context is that Lighthouse simulates a page load in a particular environment: mobile networks and mid-tier device.

Google’s documentation about Lighthouse scoring is very precise and helps a lot to understand where PageSpeed Insights rating is coming from:

Lighthouse returns a Performance score between 0 and 100. 0 is the lowest possible score. A 0 score usually indicates an error in Lighthouse. […] 100 is the best possible score which represents the 98th percentile, a top-performing site. A score of 50 represents the 75th percentile.

Google clarifies that only the items in the Metrics section of the Lighthouse’s Performance category contribute to the score. The recommendations appearing under Diagnostic and Opportunities are not part of the score.

The Performance audit in Lighthouse consists of six metrics:

Performance tab PageSpeed Insights

Each of them has its own score.
Lighthouse weights those scores to generate the final Performance rating.

The score follows a specific color-coding map:

If you’re curious to know how each metric contribute to the final score, see Google’s spreadsheet with the scoring details.

What Is a Good PageSpeed Score?

It won’t be difficult to find articles on the web promising to reveal the secret tip to score 100/100 on Google PageSpeed.

But now that you understand how the final score is calculated, it’ll be easier to understand why scoring 100/100 is incredibly tricky nowadays.

Mobile-first indexing also adds to the challenge.

Obtaining a green PageSpeed Insights score can, of course, feel rewarding, but the real question to ask yourself should be different: does that single PageSpeed score really say something meaningful about my website?

In our opinion, it doesn’t (and here’s why you shouldn’t care about your Google PageSpeed score).

There are a lot of tricks that can help to achieve a green score on PageSpeed.

For example, applying some code optimization techniques (like deferring non-critical JS files or unused CSS), could be an excellent way to improve the first paint of your pages, and obtain a green score.

But that won’t be sufficient.

The real indicator of quality for your site is its loading time.

The lower loading time your site will obtain on the six Performance metrics indicated above, the better its overall score will be.

So, PageSpeed Insights and SEO: are they related?

The mere PageSpeed score your site obtains won’t influence Google’s ranking: that number alone is not an indicator of an “SEO grade.“

So there’s not a direct correlation between how your site scores, and the position it will gain in the SERP.

But, there’s a but.

Since, as we saw, the PageSpeed score is the result of overall performance analysis on a few specific metrics, the better your score, the more you can assume your site is in good shape.

A good score is a fair indicator of the well-being of your site in the eyes of Google. But it won’t guarantee the victory in the race to the top of the SERP.

Given that Lighthouse scoring system rewards websites reaching satisfactory performance metrics, if you work on making your site fast and improving its user experience, you’ll also get better PageSpeed scores.

A faster website has higher chances of being loved by Google.
Your efforts should then be focused on making sure your website loads as fast as it can: page speed is the real ranking factor, not the score.

To learn how you can achieve this goal, you can start from the article about page speed optimization for WordPress and the errors to avoid.

Wrapping Up

In this article you learned the foundations of Google PageSpeed Insights:

Now it’s time to go ahead and learn about other aspects of this fundamental web performance testing tool:

Author's avatar

Alice is a content writer with strong experience in international customer service and empathic communication. She is an active member of the WordPress community: she loves translating WordPress into Italian and speaking at WordCamps. You can follow @Alice_Ridice on Twitter.

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