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The most important information about website speed that didnt fit anywhere else.

General

Website speed is crucial for the success of your online business. Find out why.

Why fast web

Why have a fast website

We’re introducing three new ways to keep conversions from slipping away due to a slow website. Over the years, we’ve optimized performance for dozens of projects at PageSpeed.ONE.

Expands services

PageSpeed.ONE expands services with WordPress and Shoptet optimization and SpeedCare monitoring

Bugfix 2026-02-01

Changelog: Bugfix 2026-02-01

Notes 2026-02-01

Changelog: Notes 2026-02-01

Summary of updates for the 2026-02-01 release.

Release 2026-02-01

Changelog: Release 2026-02-01

Ve verzi 4.9.1 přinášíme několik vylepšení a oprav, které zlepší vaši práci s naším monitoringem rychlosti webu.

Monitoring v4.9.1

Changelog: Monitoring v4.9.1

Metric: Third-Party JavaScript Blocking Time (3PBT). This is an internal PageSpeed.cz monitoring indicator that helps you focus on issues caused by third-party components, which can degrade the Interactions (INP) metric. (3PBT)

Third-Party Blocking (3PBT)

Metric: Third-Party Blocking Time (3PBT)

Do ads on a website push the text you’re reading out of view? Frustrated when content shifts just before you click and you end up clicking somewhere else by mistake?

Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) Metric: How to Find and Eliminate Unwanted Page Shifts

Core Web Vitals (CWV) is Google's essential set of metrics designed to reflect website loading speed and user experience. Let's explore which metrics it includes, why they matter, and how to measure them.

Core Web Vitals (CWV)

Core Web Vitals

First Contentful Paint (FCP) is a metric that measures the time from the start of page load to the moment the browser renders the first piece of content on the page.

First Contentful Paint (FCP)

The most important metrics for measuring web speed. Core Web Vitals and the others.

Metrics

Google introduces a new metric, Interaction to Next Paint (INP). In March 2024, as part of the three most important Core Web Vitals metrics, it will replace First Input Delay (FID).

Interaction to Next Paint (INP)

Interaction to Next Paint (INP) Metric: Tackling Slow Interactions

The world of Largest Contentful Paint (LCP) metrics is like adrenaline-fueled auto racing—a constant race against the clock.

Largest Contentful Paint (LCP)

Largest Contentful Paint (LCP) Metric: A Guide to Faster Web Loading

Lighthouse Performance Score (LPS) is a synthetic metric used to evaluate the speed and performance of websites, provided by the Lighthouse tool. The score is calculated from a range of metrics that simulate a user loading the page and provides an evaluative view of content speed and accessibility.

Lighthouse Score (LPS)

Lighthouse Performance Score (LPS)

We present a single number that acts as a proxy metric for Core Web Vitals on mobile and desktop. We call it the PageSpeed.cz Score (SPS).

PageSpeed.cz Score (SPS)

PageSpeed.cz Score (SPS): How we measure web speed for users?

Total Blocking Time (TBT) is a web performance metric that indicates the total time during which the page is not able to respond to user input.

Total Blocking Time (TBT)

Total Blocking Time metric (TBT)

TTFB shows the speed of the server (or infrastructure), the backend parts of the application, and also the network speed between the server and the user’s browser.

Time To First Byte (TTFB)

Prices, add-ons, and discounts for Monitoring Speed PLUS.

Pricing

Pricing for Monitoring PLUS

The most important information about website speed monitoring.

Monitoring

Watchdog monitors the speed of the pages on your site daily in Monitoring PLUS. If any key metric changes, Watchdog will alert you. This guide explains who should evaluate these alerts and how.

Watchdog: How to Evaluate

How to Evaluate Watchdog Alerts?

This article explains exactly how we collect and process speed data for websites in the app.pagespeed.cz monitoring.

How we test?

How we measure website speed?

This page lets you edit your personal details and change the default language used in Monitoring.

User Settings

Getting the setup right is crucial for accurate speed measurements—please read this text carefully.

Test Settings

Well-tuned alerts and their evaluation are central to performance budgets and the foundation of keeping your site fast.

Watchdog: Alerts

Speed Watchdog Alerts

Notes in charts help you easily evaluate the impact of changes you’ve made to your site. PageSpeed.cz PLUS monitoring also adds certain notes automatically. This article covers not only how to add and edit notes, but also our guidance on how to approach them.

Notes in Charts

Notes in Charts: The Key to Evaluating Site Speed Changes

The Domains report shows the speed status of measured domains. It is the most important report for tracking the speed development of the entire website.

Domains Report

Watchdog monitors the speed of the measured pages on your website daily. If any key metric changes, Watchdog will alert you.

Watchdog Report

The landing page, the so-called “Summary,” provides a quick view of your site’s current speed across different contexts and measurement types

Summary Report

The “Pages” report in PageSpeed.cz monitoring shows the current state and trend of the speed of the measured URLs.

Pages Report

Report “Pages”

Technical indicators on websites are often overlooked but can play a key role in improving speed and performance. In this article you will learn why it is important to monitor data volume, the number of requests, and other technical metrics, and how they can influence user metrics such as LCP or FCP. You will also discover how to use PageSpeed.cz services for effective monitoring and optimization of your site’s technical indicators.

Technical Report

The test run detail shows the results of testing a specific URL on your site.

Report “Test Run Detail”

Before you start using our tool, take a quick tour of its features.

Welcome!

Welcome to the PageSpeed.cz Web Speed Tester!

A faster website means more conversions and better business. Agencies that monitor speed keep clients happier and partnerships more stable. How can you leverage this for yourself?

For Web Development Agencies

Speed Monitoring: Why Every Web Development Agency Needs It

Thinking about website speed monitoring? Before you start live measurements, take a trial ride. 30 days, no card, no commitments. With everything the PLUS version offers.

Trial Version (Trial)

Monitoring Trial — Trial

For optimizing LCP, it’s essential to be able to find the “LCP element,” which is the largest element in the browser’s viewport for a given user. The display time of this element determines the LCP metric.

LCP Element

How to Find the LCP Element

Let’s explore what problems with the Largest Contentful Paint (LCP) metric you may encounter and how to optimize the metric as efficiently as possible.

LCP Optimization

LCP Metric Optimization

Lazy loading is a foundational and highly effective way to speed up page loads. We recommend conservatively deferring all images outside the first viewport. Be careful applying lazy loading to images that are LCP elements, as it can actually hurt speed.

LCP: Lazy-loading Images

Lazy-loading Images: A Detailed Guide

Back/forward cache (BFcache) is a browser optimization that makes back and forward navigation instant. It greatly improves browsing experience, especially for users on slower networks or devices.

BFcache

BFcache: Speed Up Back and Forward Navigation

Cloudflare acts as a gateway between your website and its visitors. Unlike a standard CDN (Content Delivery Network), it’s more than just a system for distributing images and files.

Cloudflare

Cloudflare: Your Secret Weapon for a Faster, More Reliable Website (with Checklist)

This article explains why and how to ensure element dimensions at the HTML or CSS level for elements inserted asynchronously, such as <img>, <iframe>, or JavaScript components.

CLS: How to Keep Height?

CLS: How to Keep Element Heights (Images, JS Components, Iframes…)

Is your site speed dialed in? Great, but you may not have deployed a cookie banner yet.

Cookie Banners and Website Speed

Chrome UX Report (CrUX) is a public data set provided by Google that collects and anonymously analyzes data about real user experience when browsing the web with the Chrome browser.

Chrome UX Report (CrUX)

Chrome UX Report: Real-User Speed Data from Google

A large and inefficient page structure (Document Object Model, i.e., the DOM) can significantly slow your web and negatively affect Core Web Vitals metrics. Keeping it reasonably small and as efficient as possible is essential for the overall technical performance of your project and will impact interaction response speed (INP).

DOM Optimization: The Foundation of Speed and SEO

DOM Optimization

The world of web fonts is like a beauty salon — we all want the best, but we’re not always willing to wait for the stylist to finish. And your visitors? They’re especially impatient.

Webfont Optimization

Webfont Optimization: 6 Steps to Speed Up Your Website

In PageSpeed.cz we tackle very specific issues, and we’re often surprised by how deep we go. Data compression was no exception, and we’ll dive into it in this article.

Gzip and Brotli Compression

Web Compression with Gzip and Brotli: Are You Doing It Right?

We love WebExpo and performance. And we’re big fans of Harry Roberts, whose talks and articles always go in-depth.

HTML head and speed

Get Your HEAD in Order

Know-how

React is a popular tool for building modern websites, but speed and interactivity aren’t always automatic. In this article we’ll cover key techniques for optimizing Core Web Vitals—from hydration to DOM work—and show you how to apply them to accelerate your React projects.

React Optimization

Speed Up React: 6 Proven Tips to Optimize Core Web Vitals

Using setTimeout() to optimize long JavaScript tasks that block user interactions and degrade metrics such as Total Blocking Time (TBT) or Interaction to Next Paint (INP), a key part of Core Web Vitals.

INP and setTimeout()

setTimeout: Optimizing Long Tasks in JS

Measuring speed once in a while isn’t enough. Without continuous speed monitoring you won’t uncover hidden issues, you’ll lack a data history for analysis, and you’ll easily miss trends that could cost your business conversions and visitors.

Why Monitor?

Website Speed Monitoring: Why It Matters and How to Use It Effectively

The backend plays a pivotal role in site speed optimization, because proper code- and server-side optimization can eliminate delays that often arise when processing requests and delivering content.

Backend Optimization

Backend Speed Optimization: How to Help

Let’s take a look at how to avoid CLS problems and how to remove them when they occur.

CLS Optimization

CLS Metrics Optimization

INP isn’t the easiest metric to optimize, so we’ve drawn from our experience the most common problems and how to fix them.

INP Optimization

INP Metric: 7 Tips for Optimization

Even everyday site owners can tame their images and make them work for you. Use WebP or AVIF, compress well, and don’t forget lazy loading.

Image Optimization

Image Optimization: Kilobytes, Pixels, and Milliseconds

Shoptet is fast if you don’t slow it down. We show how to optimize speed: what harms, what helps, and how to monitor your store.

Shoptet Optimization

How to speed up your Shoptet store: practical tips and common mistakes

Boost Vue.js site speed with SSR, lazy loading, and more. Faster performance means happier users and higher conversions.

Vue.js/Nuxt.js Optimization

Vue.js and Nuxt.js Optimization: Tips & Tricks

Site speed isn’t just a cosmetic detail—it’s a key factor shaping the success of your online business. Slow WordPress sites are often caused by poor plugin choices, bloated themes, or a lack of optimization know‑how.

WordPress Speed Optimization

WordPress Speed Optimization: 6+1 Tips to Delight Google and Your Visitors

Performance budgets is a methodology for keeping web page speed at the level we set. Think of a “budget” as a limit on values for different metrics.

Performance budgets

Performance budgets (web speed limits)

Psychology, conversions, and Google — why you should have a fast website too.

Why Website Speed?

Why Optimize Website Speed?

The Speculation Rules API lets us preload or even prerender another page. This approach enables near-instant loading of the page a user navigates to, improving their overall web experience.

Speculation Rules

Speculation Rules API: The Path to Instant Page Loads

Live chat is used by 20% of Czech e-commerce operators. But how do these widgets affect site speed?

Study: Chats and Speed

PageSpeed.cz Study: The Impact of Chat Widgets on Website Speed

In the PageSpeed.cz team we have several evergreen discussions, and one of the perennial questions is: Is it faster to have many small CSS files split by components, or a single large file?

Study: One CSS File or Many?

Test: Is it better to have many small CSS files or one big one?

In this article, we clarify the differences between the various types of web speed measurement. We’ll discuss synthetic data (“synth”), data from Google users (“CrUX”), and data from all users (“RUM”).

Synth vs. CrUX vs. RUM

Types of Web Speed Measurement: Synthetic, Chrome UX Report, and Real User Monitoring

Understanding the different navigation types is important because some are much faster than others. From a speed optimization perspective, the goal is to convert as many users as possible to fast navigation types.

Navigation Types

Core Web Vitals are the metrics we use to evaluate website performance in areas such as loading speed, interactivity, and layout stability. These metrics are increasingly important for delivering a good user experience, and Google uses them as a ranking factor in search results.

Web Vitals in the Browser

Testing Web Vitals in the Browser (A Developer’s Key Skill)

Page load speed is often measured in seconds. The WebP image format can be the secret weapon to trim kilobytes and push performance toward perfection.