Awesome Web Performance Metrics Awesome

Metrics to help understand page speed and user experience.

Contents

Concepts

Lab Data (Synthetic Measurements)

Make a request to your page with a tool and evaluate performance. Be sure to make it realistic (e.g. by throttling network and CPU) and reduce noise (e.g. by running multiple times).


Field Data (Real User Monitoring - RUM)

Collect performance data from real users visiting your page. Be mindful of the actual overhead, as it runs in your user's browser and watch out for browser support of more recent metrics (e.g. compared to your user-base).

Critical rendering path

The critical rendering path is everything that happens between receiving network bytes and rendering something on the screen. To optimize any rendering metrics like First Contentful Paint (FCP) or Speed Index you have to understand how the critical rendering path works.

Long tasks

The browser Main Thread that handles user input is also the one executing JavaScript (among many other things). Blocking the Main Thread for too long can make your page unresponsive.

A user perceives any visual change within 100ms as instant. Any task blocking the Main Thread by taking longer than 50ms is considered a long task (as it might make the browser unresponsive to user input).

To optimize interactivity metrics like Time to Interactive (TTI) and First Input Delay (FID) you have to understand long tasks and how to avoid them as much as possible.

User-centric metrics

Users are typically looking for visual feedback and reassurance. To measure this perceived performance (at various stages of loading) we can choose metrics that directly answer the questions below.

Apdex score

Application Performance Index, or Apdex, is a measurement of your users’ level of satisfaction based on the response time of request(s) when interacting with your website or application.


Rendering metrics

First Contentful Paint (FCP)

First Contentful Paint marks the time at which the first text or image is painted (including background images), non-white canvas or SVG. This includes text with pending webfonts. This is the first time users could start consuming page content.

First Meaningful Paint (FMP)

First Meaningful Paint measures when the primary content of a page is visible. It's essentially the paint after which the biggest above-the-fold layout change has happened, and web fonts have loaded.

Speed Index

Speed Index shows how quickly the contents of a page are visibly populated (lower numbers are better). This is done by frequently measuring visual completeness during loading. The quicker the page is more visually complete the lower the value.

Start render

The Start Render is the time from the start of the initial navigation until the first non-white content is painted to the browser display.

First Paint (FP)

First Paint reports the time when the browser first rendered after navigation. This excludes the default background paint, but includes non-default background paint. This is the first key moment developers care about in page load – when the browser has started to render the page.

Visually Complete

The Visually Complete is the time from the start of the initial navigation until the visible (above the fold) part of your page is no longer changing. (Measured using a color histogram based on video/screenshots recording).

Hero Element Timing

Hero Element Timing captures when specific elements are painted by the browser (e.g. your h1 or your hero image, etc).

Cumulative Layout Shift score (CLS)

A metric derived from the Layout Instability API. The cumulative layout shift (CLS) score is determined by calculating the sum of all unexpected (not within 0.5s of a user interaction) layout shift scores from page load until the page's lifecycle state changes to hidden.

Largest Contentful Paint


Interactivity metrics

First CPU Idle

First CPU Idle marks the first time at which the page's main thread is quiet enough to handle user input.

Time to Interactive (TTI)

Time to interactive is the time it takes for the page to become fully interactive (as in Main Thread quiet for 5s). Not to confuse with First Interactive or First CPU Idle. (Warning: one of the most confusing and misunderstood metrics).

First Input Delay (FID)

First Input Delay (FID) measures the time from when a user first interacts with your site to the time when the browser is actually able to respond to that interaction. An interaction can be when users click a link, tap on a button, or use a custom, JavaScript-powered control.

First Interactive

See First CPU Idle. WPT still calls it First Interactive but Google/Lighthouse renamed to First CPU Idle to avoid confusing this with Time to Interactive (TTI)

Consistently Interactive

See Time to Interactive (TTI). WPT still refers to TTI as Consistently Interactive but it's only available for Chrome and not surfaced on the UI (only in raw results XML/JSON).

Estimated Input Latency

Estimated Input Latency is an estimate of how long your app takes to respond to user input, in milliseconds, during the busiest 5s window of page load. If your latency is higher than 50 ms, users may perceive your app as laggy.

Max Potential First Input Delay

The maximum potential First Input Delay that your users could experience. Basically equals to the duration of the longest long task on the browser Main Thread.

Total Blocking Time


Network metrics

Network timing field data can uncover a non-optimized TLS setup, slow DNS lookups or server side processing and issues with CDN configuration. See also a separate section about measuring transferred bytes.

DNS latency

// Measuring DNS lookup time
var pageNav = performance.getEntriesByType("navigation")[0];
var dnsTime = pageNav.domainLookupEnd - pageNav.domainLookupStart;

TCP and SSL/TLS latency

// Quantifying total connection time
var pageNav = performance.getEntriesByType("navigation")[0];
var connectionTime = pageNav.connectEnd - pageNav.connectStart;
var tlsTime = 0; // <-- Assume 0 by default

// Did any TLS stuff happen?
if (pageNav.secureConnectionStart > 0) {
  // Awesome! Calculate it!
  tlsTime = pageNav.connectEnd - pageNav.secureConnectionStart;
}

Time to First Byte (TTFB)

var ttfb = pageNav.responseStart - pageNav.requestEnd;

Transferred bytes

You can measure the byte weight of your assets with a number of tools. You would normally track these Lab only as the numbers are usually the same in the Field (but be mindful of device type or geographical location specific pages).

Measuring own (and third-party) JavaScript bytes is crucial as JavaScript is the main cause of high TTI or FID values.


Other metrics

Google PageSpeed Insights score

User Timing

The User Timing API allows the developer to create application specific timestamps that are part of the browser's performance timeline. e.g. you can create a user timing mark to measure when your JS has loaded for a specific component on the page.

Server Timing

Surface any backend server timing metrics (e.g. database latency, etc.) in the developer tools in the user's browser or in the PerformanceServerTiming interface.

Frame rate

The frame rate is the frequency at which the browser can display frames. A frame represents the amount of work a browser does in one event loop iteration such as processing DOM events, resizing, scrolling, rendering, CSS animations, etc. A frame rate of 60 fps (frames per second) is a common target for a good responsive user experience. This means the browser should process a frame in about 16.7 ms.

DOMContentLoaded

window.load

License

CC0

To the extent possible under law, Csaba Palfi has waived all copyright and related or neighboring rights to this work.