How to Use WebPageTest: A Tool for Measuring Site Display Speed

Website loading speed is a critical factor for both user experience and SEO. To accurately measure and improve site performance, reliable tools are essential.

WebPageTest is a free, open-source web performance measurement tool widely used by developers and web marketing teams around the world.

This article provides essential information for team members involved in website performance improvement projects, particularly those who need to quantitatively measure and report results.

The official WebPageTest website can be accessed below.

Reference https://www.webpagetest.org

What is WebPageTest?

WebPageTest is an open-source tool for comprehensive measurement and analysis of website performance. It allows you to conduct web page loading tests that simulate real user environments from servers located in data centers around the globe.

The tool's greatest advantage is that it provides enterprise-level detailed measurement results and analysis information for free. Many Fortune 500 companies utilize it in their internal web performance optimization projects.

Reference Akamai Japan Blog | Akamai Official

Key Features of WebPageTest

Precise Environment Configuration and Highly Reproducible Testing

WebPageTest allows you to specify test environments in detail, enabling consistent performance measurements. You can simulate real user experiences by selecting specific device types (iPhone, Android), browsers (Chrome, Firefox), and connection speeds (4G, 3G).

Comprehensive Reporting and Easy Sharing

Each test generates a unique URL that can be shared with team members and stakeholders. Reports include detailed waterfall charts, visual film strips, and actionable optimization recommendations.

Advanced Automation and API Integration

The tool offers a robust REST API that integrates seamlessly with CI/CD pipelines. You can connect it to Google Sheets, Slack, and monitoring tools to establish regular performance tracking and alert systems.

Why Choose WebPageTest?

Measuring website performance accurately is challenging. For marketing teams, quantitatively reporting optimization results is crucial, but variations in measurement conditions often make it difficult to obtain reliable assessments.

Website loading speed is influenced by numerous factors including device performance, network conditions, and geographic location, making consistent measurement criteria essential.

Limitations of Google Analytics Page Load Speed

Google Analytics' site speed report collects real user experience data (RUM), but has several limitations:

• Sampling-Based Measurement: Data is collected from randomly selected samples rather than all user sessions
• Environmental Variability: Results are significantly influenced by users' device performance, network speed, and location
• Difficulty Measuring Improvements: The impact of optimization efforts can be obscured by fluctuations in user environments

This explains why you might implement site optimization measures yet see no change in Google Analytics metrics. The issue isn't that your optimizations are ineffective, but rather that the measurement method itself has inherent limitations.

Standardized Measurement with WebPageTest

WebPageTest provides consistent performance measurement through controlled testing environments. Key advantages include:

• Environmental Standardization: Tests run under identical device specifications and network conditions
• Reproducibility: Ability to conduct repeated tests under the same conditions
• Before/After Comparison: Quantitative measurement of optimization effects

Note that some variation in results may still occur due to testing time (time of day, day of week) and server load. We'll address these challenges in the best practices section later.

Understanding WebPageTest Reports

Using WebPageTest is straightforward. Simply visit webpagetest.org, enter the URL you want to test, and click "Start Test" to begin the measurement.

Interpreting Test Results

WebPageTest reports evaluate site performance using these key scores:

• Performance Score: Overall site performance rating
• Security Score: Assessment of HTTPS implementation and security headers
• Core Web Vitals: User experience metrics prioritized by Google

Scores range from A (highest) to F (lowest). If your security score is low, consider implementing HTTPS and security headers as soon as possible.

Key Performance Metrics

• First Byte: Time until the first byte of data arrives from the server
• Start Render: Time until content first appears on the previously blank page
• Speed Index: Measurement of how quickly content is visually displayed
• Document Complete Time: Time until static content (images, HTML) finishes loading
• Document Complete Requests: Number of requests made for static content
• Document Complete Bytes In: Amount of data transferred for static content
• Fully Loaded Time: Time until all content (static and dynamic) finishes loading, which may be slower if JavaScript is dynamically loading content
• Fully Loaded Requests: Total number of requests for all files including both static and dynamic content
• Fully Loaded Bytes In: Total amount of data transferred for all content

Reference Continuous Performance Improvement Starting with Webpagetest: Page Load Time Edition | azu.github.io

Other Important Performance Metrics

Additional metrics commonly used in performance measurement include:

• First Contentful Paint: Time when the browser renders the first content from the DOM, providing the user with initial feedback that the page is loading
• Largest Contentful Paint: Time when the largest/most significant content element becomes visible
• Cumulative Layout Shift: Measures visual stability by quantifying unexpected layout shifts during page loading

Creating Automated Performance Reports

For enterprise-level continuous performance monitoring, automation is essential. WebPageTest's robust API enables integration with various tools to create automated reporting solutions.

Automation with Google Apps Script and Google Data Studio

The gas-webpagetest solution automates WebPageTest measurements using Google Apps Script and visualizes results in Google Data Studio, enabling regular monitoring and comprehensive dashboard creation.

Reference Automating and Visualizing WebPagetest Performance Measurements with gas-webpagetest | Web Scratch

Free Template for Bulk Performance Testing

To streamline performance management for enterprises, we provide a free template for bulk testing multiple pages with WebPageTest.

Reference Web Page Test - Bulk Test Template | Google Spreadsheet

Template Features

• Bulk Testing: Measure multiple URLs simultaneously
• Continuous Monitoring: Stores data for 12 instances, perfect for monthly tracking
• Simple Operation: Complete measurements with just URL input and a button click
• Reporting: Automatic graph generation and trend analysis

How to Use the Template

Conclusion

This article has introduced both basic WebPageTest usage and advanced automation solutions. By implementing continuous performance monitoring and making targeted improvements, you can enhance user experience and boost your SEO effectiveness.