Fundamentals of page speed monitoring

Dr. Felix Gessert
September 8, 2023
5 min read
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Ever clicked on a webpage only to wait endlessly for it to load? It's frustrating, right? Page speed plays an indispensable role when it comes to user experience on websites.

To name just a few, performance metrics of that get negatively influenced when a page is slow:

  • Bounce rate: Users hate to wait on websites. Attention spans are shorter. If a website needs too long to load, the users will bounce.
  • SEO ranking: Google also ranks websites based on page speed through the Core Web Vitals. So fast, websites get higher rankings.
  • Conversion rates: If a website provides a good user experience without many waiting times, it's more likely that they click through the entire customer journey and convert in the end.

So there are good reasons to optimize page speed. When trying to speed up a website, it can be hard to keep track of how fast the pages are actually loading. If it's not done properly, important things that could help increase page speed might be overseen or forgotten. Therefore, in this article, the fundamentals of page speed monitoring are discussed.

But first it's important to differentiate between two words that are often used as synonyms, even if they are definitely not. These words are measuring and monitoring. Measuring means taking a snapshot of the performance metrics at a certain point of time. But this is only one value. There is no comparison if the value is good or bad. Monitoring, on the other hand, describes the observation of performance metrics over time. It also includes the highlighting of anomalies and the assessment of trends. Monitoring is the activity we want to focus on in this article.

Metric Definition Required performance
Largest Contentful Paint (LCP) The time it takes for the main content of a web page to load and become visible to the user. 75% of users should have an LCP below, 2500ms.
First Input Delay (FID)
The time from when a user first interacts with a page to the time when the browse starts processing event handlers.
75% of users should have an FID below 100ms.
Cumulative Layout Shift (CLS) CLS measures the largest layout shift based on size and distance it moves for every unexpected shift during the entire time span of the page's existence. 75% of users should have a CLS score below 0.1.
First Contentful Paint (FCP) The time from when the page starts loading to when any part of the page's content is rendered on the screen. 75% of users should have an FCP below, 1800ms.
Interaction to Next Paint (INP) The longest time it takes for a website to respond to user interactions such as clicks, taps, or keystrokes. At the Beginning of 2024, it will replace the FID. 75% of users should have an INP below 200ms.
Time to First Byte (TTFB) The time it takes for a user's browser to receive the first byte of data from the web server after they click. 75% of users should have a TTFB below 800ms.

The most important KPIs for page speed monitoring

This article covers six so-called web vitals defined by Google, which are also relevant for SEO. The first three metrics in the table below are called “Core Web Vitals” and are the most important ones.

Different tools for monitoring page speed

There are three different categories of monitoring tools. The first categories are tools that evaluate pages based on lab results. That means, the data consists of performance information gathered within a controlled setting, utilizing predetermined device and network configurations to assure comparability. Lighthouse and Speed Kit's Page Speed Analyzer fall into this category. 

It's important to mention that those applications only show the before mentioned snapshots. An external space would be necessary to monitor the data over time. Because of the controlled environment and the possibility to generate reproducible results, Lab data is really helpful for debugging performance issues. Additionally, there are many free tools to conduct lab tests, while most of the tools that work with real life data have to be purchased. On the other hand, lab results might not reflect real life user experience, bottlenecks and KPIs since the used setup is predetermined and standardized.

The second category contains tools that use real user data but not cover all the data available. The Google CrUX report is an example of this category. Through the looker studio application, it's possible to examine page speed data over time. Page Speed Insights uses lab data as well as real user data. But tools in the second category only take parts of the real user data into account. For example, the CrUX report only includes data from chrome users who accepted the cookies and are logged into their Google accounts.

The third category contains Real User Monitoring (RUM) tools. Those consider all user data and allow monitoring page speed over time. Click here to learn more about the difference between CrUX and RUM data. Some tools also implement the CrUX report in the RUM dashboard, so CrUX and RUM data can be compared. This is extremely useful, for example, for fixing Layout Shifts. Tools like Debugbear allow this, but also Speed Kit will provide this for its customers in the future.

RUM Tools have the advantage that the real-world user experience is captured with all its factors. Through this, it's also possible to create connections to business KPIs. On the other hand, sometimes the amount of data is not high enough to calculate all the necessary metrics. Also, real-world influences make reproducible results and debugging difficult. The best is to use all categories of tools and compare the results to fully grasp the performance of the website.

Running page test experiments

Finally, it also has to be clear if the changes made to optimize the website were successful. The best way to do this is to conduct a page speed experiment. The definition of an experiment is that there are two groups. One gets a treatment. The other is a control group. All other variables are kept constant. That way, it's certain that if there was a performance change resulting from the treatment, and it's possible to statistically evaluate the experiment with A/B test statistics.

One advantage of Speed Kit is that its impact can be tested through experiments. It's possible to create different user groups, so there can be a treatment and control group while both groups experience every other event on the website, meaning holding all other potential variables constant. Therefore, its always sure that Speed Kit has the promised effect.


This article has shown the differences between measuring and monitoring, what the most important page speed KPIs are, what different categories of tools exist and what the best way is to determine if the changes on a website had a positive effect on page speed. With these insights, fundamental knowledge about page speed monitoring is provided that can be extended through the links in this article.


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