Web performance metrics and their significance

Anton Ioffe - October 29th 2023 - 8 minutes read

Welcome to a comprehensive exploration of web performance metrics in the realm of modern web development. As we journey together through this article, you’ll discover the intimate relationship these metrics bear with user experience, the ins and outs of some critical ones like Time to First Byte (TTFB) and Largest Contentful Paint (LCP), and the pitfalls that developers often stumble upon. You’ll also learn how to master the art of creating custom metrics, their potential benefits, and drawbacks. Lastly, we'll delve into the underappreciated impact of these performance metrics on SEO rankings. Get ready to enhance your knowledge and transform your perspective on web performance metrics, their significance, and implementation.

The Impact of Performance Metrics on User Experience

Optimizing web performance metrics facilitates a superior user experience—key to boosting user retention and site engagement. Serving as a gauge to measure how your site resonates with its users, these metrics should strike parity with other crucial facets, such as aesthetics or functionality. Notably, they play a substantial role in overall user approval, as dwindling performance often triggers higher bounce rates and slumps in conversions.

Beginning with Core Web Vitals, Google's brainchild, they equip developers with a sturdy foundation to evaluate site performance from a user-centric viewpoint. Consisting of three core metrics—Loading, which measures how long it takes for the main content of a webpage to load; Interactivity, indicating the time from when a user first interacts with a page, for example, when they click a link, to the time when the browser is able to respond to that interaction; and Visual Stability, meant to prevent annoying and unexpected movements of page content—they strongly influence the users' encounters. Good Core Web Vitals parameters result in a fast and responsive website, instilling user engagement and repeat visits.

Moving on to Time to First Byte (TTFB), it quantifies the time elapsed from a user's request for your webpage, till the first chunk of data is received—acting as an initial yardstick of your server's responsiveness. Streamlining TTFB is crucial to user satisfaction, as it directly casts an influence on users' perception of your site's speed and actual throughput.

A final performance metric worth highlighting is the Apdex score, representing user satisfaction based on the website's response times. It is a score calculated using the formula (Satisfied Count + Tolerated Count / 2) / Total Samples where Satisfied Count are the number of responses considered satisfactory, Tolerated Count are the number of responses considered tolerable and Total Samples are the total number of samples collected. A high score denotes swift response to users' interactions, fostering a more gratified and engaged audience. Thus, performance metrics analysis followed by its optimization serves as a cogent approach towards enhancing user experience and overall contentment. Adopting users' perspective and implementing learned insights deliver a website that is not only engaging but also seamless for your users to interact with.

Unraveling Key Performance Metrics

Time to First Byte (TTFB) is the measure of the responsiveness of a web server. This metric is essentially the delay between the client making the HTTP request and the server sending the first byte of the response. It's an indicator of the server's speed and performance, and by extension, affects the overall performance of the web application. TTFB is closely linked to server-side processing, as complex computations or database queries inevitably increase its value. It's thus important to limit server-side processing as much as reasonably possible to boost TTFB. Among typical problems, developers often experience bottlenecks linked to inefficient database queries or unoptimized back-end code.

First Contentful Paint (FCP) and Largest Contentful Paint (LCP) are key millstones in the browser's rendering process. Specifically, FCP measures the time until the browser renders the first piece of DOM content after a user navigates to your page, while LCP tracks when the largest DOM element is visible. A lower FCP means the user perceives the page to be loading quicker, and reducing LCP ensures that the user sees the most meaningful content faster. Their optimization involves minimizing the time spent parsing, compiling, and executing Javascript, as well as other factors such as render-blocking resources and slow server response times.

First Input Delay (FID) and Time to Interactive (TTI) are interactivity metrics, linked to how a page responds to user interaction. FID quantifies the delay between the user's first interaction and the browser's response to it. TTI, on the other hand, gauges the time it takes for a page to become fully interactive. Poor FID and TTI values often correspond to long tasks, excessive JavaScript execution, and other bottlenecks. Consequently, strategies to improve FID and TTI values typically involve careful organization and management of JavaScript on your page – chunking long tasks, reducing JavaScript execution time, and so on.

Cumulative Layout Shift (CLS) measures visual stability, specifically quantifying the amount of unexpected layout shift across the lifespan of a webpage. A low CLS value indicates stable visuals, enhancing the user's experience by eliminating unexpected or jarring shifts in the layout. The optimization of this metric often involves issues with images or ads without dimensions, dynamically injected content, and web fonts causing FOIT/FOUT. Providing size attributes for media elements, avoiding the insertion of content above existing ones, and the use of font-display, are some common ways to improve CLS.

Custom Metrics – Creation and Importance

In modern web development, developers aren't limited only to pre-existing performance metrics. They can also create and implement custom metrics that are tailored specifically to their websites. These custom metrics offer the edge of granular insights, allowing you to track and measure aspects that particularly matter to your site's performance and user experience. They amplify the scope of analysis, uncovering unique bottlenecks that may not be captured by standardized metrics. For instance, if your website relies heavily on third-party APIs, a custom metric that measures the latency of these APIs can be more insightful and relevant than a standard load time metric.

Creating custom metrics involves identifying a specific aspect of your website's performance you wish to measure. This could relate to user experience, backend efficiency, or frontend load time, amongst others. Once identified, determining what exactly to measure becomes critical. In JavaScript, manually logging performance measurements can be accomplished with the performance.mark() and performance.measure() APIs. For example, to measure the time taken for an API request to complete, a mark can be initiated at the start and end of the request, and the time duration can be measured.

However, any custom metric comes with inherent pros and cons. The key advantage of custom metrics is the granularity and relevancy they provide, focusing on an aspect that directly affects your website. This specificity can reveal insights that a general metric might overlook, leading to more effective optimizations. On the downside, custom metrics can get complex, requiring in-depth understanding and careful implementation to ensure they accurately reflect what they're intended to measure. They can also add additional overhead to the monitoring process, and determining the appropriate 'good' or 'bad' range for a custom metric—referring to the acceptable and unacceptable values—may not be straightforward.

Nonetheless, if implemented correctly, custom metrics can be a powerful tool in your web performance arsenal. They allow you to quantify specific user experiences and track improvements over time, which can help inform and direct your optimization efforts. However, the fundamental rule remains applicable, whether using custom or standard metrics. That is to focus on metrics that directly impact user experience rather than obsessing over achieving ideal numbers.

Pitfalls in Performance Metrics

One common pitfall developers face when dealing with performance metrics is overemphasis on metric scores without considering the real user experience - the metrics are tools, not goals. Such an approach can lead to over-optimization and neglect of user-centric development. For instance, optimizing solely for a perfect Lighthouse score might not necessarily make users happier if it negatively impacts usability. A balanced outlook is essential that involves regularly measuring performance while keeping user experience at the heart of optimization efforts.

Another common mistake is interpreting variability in performance scores as changes in site performance. Performance scores can fluctuate significantly due to varying network conditions, device performance, location, or even dynamic site content. As witnessed with Lighthouse analysis, even though it provides an overall score, treating performance as a score distribution rather than a single static number presents a more realistic, well-rounded view of site performance.

// Incorrect approach: Obsession with perfect score.
const lighthouseScore = getLighthouseScore(); // some function to get Lighthouse score

if(lighthouseScore < 100){
   // Over-optimizing to reach a 'perfect' 100 score

// Correct approach: Balance between score and user experience
const lighthouseScore = getLighthouseScore();
const userFeedback = getUserFeedback(); // some function to retrieve user feedback

if(lighthouseScore < 85 && userFeedback.isNegative()){
   // Balance optimization with maintaining/enhance user experience

Frequently, developers take a bottom-up approach towards performance optimization, starting from elementary metrics like page load time, then proceeding to more nuanced metrics such as Core Web Vitals or custom metrics. While this seems logical, it might make more sense to approach it from the top-- down. Prioritize optimization efforts that have the most impact on user experience, then work your way into the more technical aspects of performance optimization.

Lastly, there can be a tendency among developers to merely rely on standard metrics and reports instead of customizing their performance monitoring approach. Standard metrics offer a general performance picture but might not completely capture the unique characteristics of your website. While creating custom metrics might seem complex, they can offer more insightful, precise information tailored to your needs.

// Incorrect approach: focusing solely on standard metrics
const pageLoadTime = getPageLoadTime(); // function to get page load time

// Analyze based on page load time. Yes, it's a good standard metric. But it might not cover all your bases.

// Correct approach: Custom metrics to cater to unique needs
const myCustomMetric = calculateCustomMetric(); // function to calculate custom metric based on unique website needs

// Analyze based on custom metric, gain tailored insights.

In conclusion, while performance metrics are a highly valuable tool in modern web development, it's essential to avoid common pitfalls. A proper understanding, coupled with a holistic approach, calibrates the focus on improving user experience rather than just scores.

Performance Metrics and SEO Ranking

The correlation between web performance metrics and Search Engine Optimization (SEO) is often overlooked. However, search engines, primarily Google, use various performance metrics as part of their ranking algorithm. These metrics are directly related to the user experience, making it pivotal for developers to factor this into their optimization strategies.

One of the performance metrics affecting SEO ranking is page load time. Companies such as Amazon have found a direct correlation between page load time and increase in sales. A faster website leads to less frustrated experiences, consequently translating to return visits. Even a reduced page load time by 100 milliseconds can increase sales by 1%, which for companies like Amazon can mean billions in additional revenue.

Google's preference for performance metrics as a ranking factor is substantiated by their introduction of Core Web Vitals and Lighthouse scores. Core Web Vitals measure page experience from the user's perspective, providing insights into areas such as loading performance, interactivity, and visual stability. Similarly, Lighthouse scores provide actionable and pragmatic suggestions by aggregating several audits like First Contentful Paint, Speed Index, Largest Contentful Paint, Time to Interactive, Total Blocking Time, and Cumulative Layout Shift.

Therefore, it is essential for developers to not just focus on traditional SEO practices of keyword optimization and backlinks but also give due importance to site performance metrics. By optimizing these metrics, developers can enhance their site's SEO ranking and visibility, thereby leading to better user experience, increased conversions, and ultimately, a successful and profitable website.


In this article on web performance metrics and their significance in modern web development, the author explores various metrics like Time to First Byte (TTFB), Largest Contentful Paint (LCP), and Core Web Vitals that impact user experience and SEO rankings. The article emphasizes the importance of optimizing these metrics for improved user engagement and site performance. The author also discusses the creation and implementation of custom metrics for a more tailored analysis. A challenging technical task for readers is to identify and prioritize optimization efforts that have the most impact on user experience, rather than solely focusing on standard metrics.

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