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Frontend Performance Measuring, KPIs, and Monitoring

Frontend Performance Measuring, KPIs, and Monitoring

Frontend performance matters for user experience, conversion, ad placements, and good old SEO. To build fast websites, you need to have the performance metrics and KPIs defined, monitor, understand, and act upon them.

Frontend Performance Measuring, Monitoring & KPIs

Core Web Vitals, INP, LCP, Cumulative Layout Shift, FID,, Google Lighthouse, Pagespeed Insight, CrUX. We're sure you've stumbled upon one of these in your quest to improve front-end performance, right?

But how do you measure performance? And why do you need to be data-driven and continuously monitor, optimize, and build a performance testing and optimization culture?

Let's explore that. Let's go millisecond hunting.

Determine Your KPIs

Frontend performance KPIs, or Key Performance Indicators, are the first thing you must establish before measuring or improving. With your KPIs in place, you have defined the game's rules in your organization, selecting a common language that you use in frontend performance subjects.

I highly recommend that you choose KPIs that are common in the industry. With that, you get the benefit of a vast selection of measurement tools available, and you focus your attention on the same metrics as Search Engines since they factor in speed as part of the page ranking.

The best option for this at the moment is the Core Web Vitals. Core Web Vitals do not encompass all the available frontend performance metrics, but they include the most important ones that can be reliably measured.

Core Web Vitals

In May 2020, Google announced new metrics to measure the quality of user experience on the web by quantifying the Core Web Vital metrics. Measuring core web vitals is the critical metric added to the various tools to measure frontend performance and page speed.

The Core Web Vitals consist of (as of the time of this article) three different metrics:

You can identify these three metrics as a part of a performance audit done with the popular online performance tool Pagespeed Insights.

Behind the Scenes of All the Performance Tricks and Hacks

Field Data vs. Lab Data

The three Core Web Vital metrics are represented in the image above. Make a note of the heading Field Data. This indicates that these performance audits have been pulled from actual users on the target website (collected automatically when using Google Chrome). These numbers reflect real experiences that the users are getting and are also what Google uses as part of the page ranking algorithm.

When measuring performance, there are a lot of factors that influence the results.

  • The device (laptop, cell phone)
  • The network type (wifi, cellular, cable)
  • The network health (at home or a busy café)

A rule of thumb here is that the device's speed and network are a primer for the performance result. You can get vastly different results for the same website if you test with another device or over a different network setup.

On the other hand, Lab Data is not based on actual user reporting but on a single test run using a simulated device on a throttled network.

Lab data is not what your users see and is not used by Google in their ranking algorithm. It is only helpful for developers who can use the results of a single test to check if changes in the code had any impact on performance.

In other words, it is super helpful for the developer in analyzing how changes to the code affect frontend performance, but not at all relevant when telling the world how fast your site is.

But, It’s Not Just About Core Web Vitals

When we talk about Core Web Vitals, for Google, it is not just about them. Core Web Vitals are a part of the Page Experience, and it is this that you must pass to improve your rankings.

On top of that, not all CWV reports are created equal. What we mean by that is that when it comes to Core Web Vitals for SEO, the only place to check them is Search Console, which uses data from the Chrome User Experience Report, or CrUX, the official dataset of the Web Vitals program (we'll talk some more about it later on).

Finally, don't be missed, thinking performance is all there is. Google Search always seeks to show the most relevant content, even if the page experience is sub-par. Performance matters for many things.

Performance KPI 1: Time to First Byte (TTFB)

Time to First Byte (TTFB) is an indicator of server efficiency. A delayed TTFB might suggest server congestion, inefficient server-side scripting, or network issues. Ideally, the TTFB should be under 200 milliseconds for a server in close proximity to the user. However, factors like CDN usage can impact this.

Our dedicated TTFB page linked above has more on how to improve TTFB.

Performance KPI 2: Largest Contentful Paint (LCP)

As the most influential of the Core Web Vitals, the Largest Contentful Paint (LCP) makes up 25% of the Lighthouse score, making it the number one frontend performance KPI. It measures the time it takes for the most significant portion of the visible screen to be displayed and painted on the user's device. It directly correlates with user experience.

In general, to improve LCP, you'd optimize and compress images, leverage browser caching, prioritize above-the-fold content, reduce server response times, etc., depending on the use case OFC. Our dedicated LCP page linked above has more on how to improve LCP.

Performance KPI 3: Cumulative Layout Shift (CLS)

Claiming 15% of the total Lighthouse performance, Cumulative Layout Shift is now established as an essential metric that you should not ignore. It measures all the layout's small and large "jumps" during the page load phase. You want to keep this score as close to 0 as possible, as a score of 0 indicates that the layout did not change at all. CLS measures the instability of content.

Some optimization techniques can help you improve CLS scores, like specifying dimensions for images, videos, and other media elements, using CSS aspect-ratio property, etc. Our dedicated CLS page linked above has more on how to improve CLS.

Performance KPI 4: First Input Delay (FID)

First Input Delay measures the delay the user experiences upon their first input on the webpage. Input in this context is clicking on a button or link or using a javascript-powered feature. Most users would not enjoy interacting with your web page, only for nothing to happen afterward. A score of 0 is the best you can achieve here, with no measured delay for the user. You want to keep it under 100ms to stay in the green performance zone because high FID suggests a sluggish interactive experience.

Interestingly enough, FID is not used in the Lighthouse performance scoring. This is because the Lighthouse Lab tests cannot correctly simulate what actions users might be doing, never interacting with the page, thus making it impossible to measure FID.

To improve FID, look at another metric on the Lighthouse Performance Report: Total Blocking Time (TBT).

Google is making plans to sunset FID as part of Core Web Vitals. Don't worry; it will happen in March 2024 when they plan to replace its importance with the newly introduced metric Interaction to Next Paint (INP). However, we do suggest you start preparing for it. Use Search Console reports INP data. Improving Interaction with Next Paint can be achieved through various optimization techniques, and some of them we've mentioned in our INP post are already linked👆

Total Blocking Time (TBT)

Total Blocking Tim3 (TBT) makes up 30% of the Lighthouse performance score. However, it is not part of the Core Web Vitals and is not recorded as part of the Field Data for your website. On the other hand, it is a good indicator when doing one-off Lighthouse tests for the First Input Delay.

TBT measures the time a user is blocked from doing interactions at the critical page load time and is a metric that you want to get close to as 0 as possible.

A score of 0 indicates that the user is not blocked, whereas a score of, let’s say, 1000 indicates that there was an accumulated time of 1 second where the user was blocked. Not a very good user experience. You typically want to keep this number below 300 to ensure a happy journey for the user.

Comic strip illustrating the frontend performance metric Time to Interactive

Other Performance Metrics

These three metrics are the most important when defining your performance KPIs. There are a bunch of other metrics (speed index comes to mind), and they all serve a common purpose: to improve the experienced speed for the end-user. Frontend performance is an essential factor in digital product design, and the metrics described above measure just that.

When optimizing frontend performance, you also need to monitor your backend performance. For example, with a headless commerce approach, you must ensure your back end has a fast ecommerce API.

How to Measure Frontend Performance?

So, we talked about the Field and Lab data. Now, let’s see which tools you can use to measure frontend performance.

Pagespeed Insights

As the most accessible tool, Page speed insights is web-based and uses Lighthouse behind the scenes. Anyone in your organization can use it; it is an excellent place to start.

Chrome Devtools

Open Chrome dev tools, go to the Audits panel, and run a test with the Performance checked. It uses Lighthouse in the background and gives back a range of metrics, with the Core Web Vitals taking center stage.

Use this instead of Pagespeed Insights if you desire an even more detailed report, which is useful for local development.

Search Console Report (CrUX Data Set)

If you haven't done so, add your website to Google Search Console. It is a free Google tool that helps you stay on top of your website's SEO performance, i.e., you can track and measure for which keywords you rank and with which URLs and also diagnose technical SEO issues like CWV and crawability of your website.

As we already mentioned, if you are analyzing your CWV for SEO, this is the place to do it. The data for the Core Web Vitals report comes from the CrUX report, and this is the data that is then used in Search to influence rankings.

Search Console shows how CrUX data influences the page experience ranking factor by URL and URL group, meaning it considers URL-level CWV data when deciding where to rank you in SERPs.

If you want to dive into how your website performs, visit It is an online service that provides page tests on a fleet of servers located in different places worldwide. The audit reports you get back show you with incredible detail all the behind-the-scenes of a page load and can be beneficial if you want to debug a particular frontend performance problem.

It is also convenient when you want to perform tests where you are interested in the metrics measured from a particular region around the globe.


GTMetrix is another excellent website performance analytics tool. It's built to help you analyze the performance of your website and provide you with a list of actionable recommendations to improve it—a bunch of cool features like Speed Visualization, Waterfall Chart, and even Video for paid users.

Web Vitals JavaScript Library

If you want to track your site visitors, I recommend the web-vitals library. It’s easy to use and integrates well with whatever frontend solution you might be running:

import {getLCP, getFID, getCLS} from 'web-vitals';


The data from the user timings API can be collected for all your users and sent to an analyzing tool, such as Google Analytics. From there, you will get a crystal clear image of how your site is performing for your user base, and you can get valuable information in reports by correlating measured frontend performance with things like conversion rate.

What About Frontend Performance Monitoring?

Frontend performance monitoring is crucial as it ensures users receive a seamless experience when interacting with web applications. Monitoring needs have surged with the increasing complexity of modern websites and applications. Downtime or performance degradation can have severe consequences, making frontend monitoring indispensable.

Frontend Performance Monitoring Tools You Can Use

There are several performance monitoring tools tailored for frontend web applications, and Google Lighthouse is one of them. It doesn't just stop at performance monitoring, however. With it, you can conduct audits that cut across several dimensions like performance, accessibility, SEO, and even best practices for progressive web apps.

Then, there is Sentry, an open-source tool that provides engineering teams with tools to detect and solve user-impacting bugs. With features like transaction tracing and performance views, Sentry is a robust choice for frontend monitoring.

We should mention Pingdom and Sematext as well. Pingdom offers uptime monitoring with features like SSL and real-user monitoring with customizable alerts and detailed performance reporting. Sematext provides a comprehensive suite of monitoring tools with features like real-time alerts, support for major frameworks, and unified log management.

Installing and configuring performance monitoring tools usually follows a standard procedure, although specifics might vary based on the tool.

Navigating a Transaction Summary

A transaction summary in frontend performance refers to an aggregated report capturing crucial metrics and details of a user transaction on a web application. It provides a holistic view of a specific user interaction or series of interactions on a web application or site through:

  • Header Information shows the URL, HTTP method (like GET or POST), status code (like 200 for success), and overall time taken.
  • Waterfall View offers a step-by-step breakdown of the request and response process. It helps identify bottlenecks in content loading or third-party scripts.
  • Trace Details may include information about JavaScript function calls, AJAX requests, and other client-side operations.
  • Metadata holds contextual data about the user's browser, location, device type, etc.

Transaction Summary provides a granular view of user interactions, helping teams understand, analyze, and optimize the user journey.

Troubleshooting a Performance Issue

Performance issues can arise due to a myriad of reasons. Troubleshooting them should involve a systematic approach to identifying and rectifying these. Usually, it is done in these steps (though not necessarily):

  1. Identify the Issue. First, review performance metrics to pinpoint potential problems. Is it a server-side issue or content rendering? Then, compare current metrics with historical data to spot discrepancies.
  2. Check the Usual Suspects. Third-party scripts, images and media files, JavaScript, and CSS usually influence performance. Making sure they are in order is crucial. Check our frontend performance checklist for more info.
  3. Check Browser Caching. Is there a caching strategy in place? Check the settings.
  4. Check Database and Server Performance. Bad database optimizations and server configurations can bottleneck performance.

Adopt Performance First Mindset

Optimizing for frontend performance is not something you do once and don’t have to worry about it again. It’s an ongoing effort that requires commitment from everyone involved. Browsers evolve, devices get faster, metrics change, and how we craft websites changes.

What never changes, though, is the importance of a friction-free user journey, where you put the user first and never compromise on performance. On top of that, performance has become one of the search ranking signals, which means it is effectively a part of your SEO efforts (especially for eCommerce SEO).

Remember, milliseconds matter🤘

You Should Also Read👇

Behind the Scenes of All the Performance Tricks and Hacks

Behind the Scenes of All the Performance Tricks and Hacks

Putting those frontend performance checklist suggestions to work.

The Cost of 3rd Party Scripts: Understanding and Managing the Impact on Website Performance

The Cost of 3rd Party Scripts: Understanding and Managing the Impact on Website Performance

3rd party scripts are also notorious for hurting your website’s performance in addition to posing security risks. With that being the case, let’s look at the cost of third-party scripts and how to manage their impact better.

Keeping Websites Fast when Loading Google Tag Manager

Keeping Websites Fast when Loading Google Tag Manager

Google Tag Manager can influence your overall website performance just as any other external 3rd party script. Luckily, there are a couple of things that you can do in that regard.