How PageSpeed Insights Works (Ultimate Guide)

Google’s PageSpeed Insights is a free and powerful tool to check websites performance. You can see how fast a website is, what issues it has, and how well it performs in general.

In this ultimate guide, I will explain how it works, what each metrics means, and how to use it to improve the performance of the website.

What Is PageSpeed Insights?

PageSpeed Insights website

PageSpeed Insights is a free website performance checker tool from Google. It checks websites for different metrics and gives a score from 0 to 100. There are three categories:

  • Bad – This is a category when a website gets score between 0-49. This means taht there are big issues and you must solve these problems.
  • Needs Improvement – The second category is between 50-89. These are important issues too, but are easeir to solve.
  • Good – The last category is when everything is perfect and it’s when the score is between 90-100.

You get detailed and explained information about a website and you can see, understand, and solve the problems. It shows render-blocking resources, how images are optimized, bad script, and other important information.

This tool checks website performance for desktop as well as for mobile. Most of the time desktop versions have much fewer issues than mobile. And because mobile performance is crucial for Google, try to improve it first.

A high score is very important. It’s critical for user experience, ranking, and SEO. If you get a bad score in tests, it will be harder to rank in Google SERP.

How To Use Google Insights

It is very easy to use PageSpeed Insights and there are two ways you can check a website’s performance with this tool. You can use its website or you can install the Google Chrome extension.

If you go to a website, you can just paste a website’s URL, click Analyze button and in seconds it will show the performance score and different results. You can see both – desktop and mobile scores.

But the faster way is the Chrome extension called Lighthouse. Just install the extension and go to the website you want to check. Choose the extension icon, click the Generate Report button and it will show the results.

Also Read: Ultimate Guide To Search Console

Also, the extension is better because it checks websites for other metrics. It will check the website for speed but also for SEO, accessibility, and best practices.

So, with this extension, you can see the whole picture. It is a great way to see issues and solve them.

Understand The Metrics

To properly use PageSpeed Insights, first, you need to understand what each metrics means, what is a good score in each section, and how to improve your website’s score.

There are seven important metrics you should know and understand. It’s better to have a good score in each metric but some metrics are crucial.

Core Web Vitals

Google core web vitals

Although all seven metrics are important and you need to improve the score in each metric, there are three of the most important ones. It’s called Core Web Vitals and contains Largest Contentful Paint, First Input Delay, and Cumulative Layout Shift.

Largest Contentful Paint

Largest Contentful Paint is the first important metrics for Core Web Vitals. LCP measures how fast the website renders the biggest element like text or image. Most of the time it’s an image that has issues here.

The threshold for LCP is 2.5 seconds and many websites struggle with this metric. And the frequent cause of problems is unoptimized, uncompressed images. Sometimes big text and headings cause problems so don’t use too big texts.

To have a good LCP score, compress and optimize images for your website. Also, you can convert images to WebP format. It reduces image size and it loads faster.

First Input Delay

First Input Delay is new to PageSpeed Insights and it’s the second metric in Core Web Vitals. FID measures how fast a website responds to interactions. For example, if a visitor clicks a button, how fast it will take a visitor to the desired page.

For Google, FID is important so you have to try to have a good score. A good score is below 100ms and if it’s above 300ms, it means there are problems. And good FID performance depends on various things.

Good and fast hosting is critical. If you have a WordPress website, choose fast, lightweight, and optimized themes and plugins. Also, Try to build websites with good UI and UX and definitely optimize them for mobile.

Cumulative Layout Shift

And the third Core Web Vitals metric is Cumulative Layout Shift. CLS is how specific elements behave on website loading. It’s all about visual stability.

For example, you go to a website, see a button, and want to click on it. But it jumps and you see an ad or other section. That’s a bad CLS experience.

Although it’s the easiest to improve, many famous and traffic-heavy websites have issues with CLS. Most of the time the reason for bad CLS is ads. A good score for CLS is 0.1 seconds.

If you want to use ads on your website but don’t want to have a bad CLS score, first choose a good theme. Secondly, try to use high-quality ad platforms.

Non-Core Web Vitals


Other than Core Web Vitals, Google’s PageSpeed Insights also measures websites for additional four metrics. These metrics are First Contentful Paint, Time To Interactive, Speed Index, and Total Blocking Time.

First Contentful Paint

First Contentful Paint behaves the same way as LCP. But instead of the biggest element, it measures the rendering time of the first element. It can be a featured image, heading, or other texts and images. The excellent score is 1.8 seconds.

FCP and LCP are almost identical and improving and optimization tactics are the same. Optimize images, compress them, don’t use too big and bold texts, and use good themes.

Time To Interactive

Another important PageSpeed Insights metric is Time To Interactive. TTI is how fast a page becomes fully interactive. Full interaction means when the page displays useful content, event handlers are registered for most elements, and response time to interaction is below 50 milliseconds.

A good TTI score is below 3.8 seconds. To have a good score in TTI, optimize Core Web Vitals, use good themes, plugins, and hosting is also critical. Well-optimized websites don’t have issues with TTI.

Speed Index

Speed Index is another metric in Google Insights. It measures how quickly the content is displayed during the page load. A good SI score is below 3.4 seconds and in general, it’s not hard to have a good score.

If a website is optimized for Core Web Vitals, there is a high chance of a good SI score. To improve your Speed Index score, try to minimize main thread work, and decrease JavaScript execution time.

Total Blocking Time

The last and important metric is Total Blocking Time. TBT measures the time that a page is blocked from various user inputs such as mouse clicks, keyboard presses, etc. A good TBT score is 0.2 seconds.

The main cause of TBT problems is render-blocking scripts. Most of the time it’s JavaScript code or CSS files. To avoid low scores, try to optimize your websites and use good caching plugins to minimize or optimize numerous scripts.

Many badly written WordPress themes have low scores in TBT. So, to avoid it, try to find trustworthy, good themes, or plugins.

Performance Result Sections

After you know all the metrics, now let’s understand the different sections of PageSpeed Insights and what they mean. Some sections show the score of various metrics and if a website passes the test. Other sections show what are the issues and how to solve those problems.

The first two sections – Field Data and Lab Data – shows if a website passed the test and gives the score. In the other two sections, we see the problem details and solution suggestions.

Field Data

Google insights field data

The most important information is in Field Data. It measures websites for Core Web Vitals and FCP. Field Data measures websites performance in the last 28 days and calculates the average score. If a website passes the test, everything is great.

Field Data has detailed information. You can see the average time for each metric. Also, it shows what score your website had on every page load in percentage.

Lab Data

Google insights lab data

Lab Data is similar to Field Data yet different. First, it measures websites for other metrics. Second, it’s not real data. Instead, Lighthouse just analyzes and calculates the estimated performance of a website.

But don’t get it wrong. It is also very important for website performance. Lab Data shows metrics that are not in Field Data such as TBT, TTI, SI, etc. When you optimizing a website, these metrics are quite useful.


In the next two sections, PageSpeed Insights show you detailed information about all issues. First is Opportunities that can help you to improve load speed. However, these issues have no direct effect on the general performance score.

For example, here you can see messages such as “Reduce unused JavaScript“, “Properly size images“, or “Avoid multiple page redirects”. And it will show which scripts cause problems or which images need optimization.

This is a great tool to identify problems and try to solve them. Also, the Google speed test offers suggestions for every issue which makes problem-solving easier.


And the last section, Diagnostics, will display more information about various performance issues. Here you will find more techy side issues. But with its recommendations, you can find the solution easily.

Some issues you can find here are enormous network loads, excessive DOM size, JavaScript execution time, image width and height problems, and more.

Bear in mind, that desktop and mobile versions may have different issues. For the most part, the mobile version has more issues than a desktop version. Also, in general, desktop versions get higher performance scores than mobile versions.


Google’s PageSpeed Insights is the best free tool to analyze a website’s speed and performance. It’s very simple to use. You can test websites from the site or use the Chrome extension.

Google speed test provides the whole picture of your website’s performance. You get comprehensive information about Core Web Vital issues, bad script, image and caching problems, etc.

And from this analysis, PageSpeed Insights calculates and gives the overall score to the website. Try to optimize a mobile version and automatically, the desktop version will have a better score.

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