Optimize Critical Path Rendering For Mobile CWV Improvement

Improving website performance, especially on mobile devices, is crucial for user experience and SEO. This article delves into optimizing the Critical Path Rendering (CPR) to enhance the Largest Contentful Paint (LCP) metric, a key component of Core Web Vitals (CWV). We'll explore a real-world scenario focusing on a specific website and the strategies employed to reduce LCP, specifically on mobile.

Understanding the Core Web Vitals Challenge

In the realm of web development, Core Web Vitals (CWV) are paramount. These metrics, which include Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS), provide insights into the user experience of a webpage. Focusing on LCP, it measures the time it takes for the largest content element (like an image or a block of text) to become visible within the viewport. Improving LCP directly translates to a faster perceived loading time and a better user experience. For mobile users, where network conditions and device capabilities can vary significantly, optimizing LCP is even more critical.

The specific challenge we're addressing involves a website, https://add-cwv-check--bbird--aemsites.aem.page/, where the LCP element was loading too slowly on mobile devices. This issue was flagged as high priority due to its potential impact on user engagement and SEO rankings. The goal was to reduce the LCP time by 300-500ms, a significant improvement that could noticeably enhance the user experience. The crux of the problem lay in the Critical Path Rendering (CPR), which refers to the sequence of steps a browser takes to render a webpage. Optimizing this path involves prioritizing the loading of critical resources and deferring non-essential ones.

The Importance of Critical Path Rendering

The Critical Path Rendering is the sequence of steps the browser undertakes to render the initial view of a webpage. This process includes parsing HTML, constructing the DOM (Document Object Model), parsing CSS, building the CSSOM (CSS Object Model), and then combining these to create the render tree, which is finally used to paint the page. Any delays in this path can significantly impact the perceived loading speed and, consequently, the LCP.

Optimizing CPR involves identifying and prioritizing the resources needed for the initial render, such as critical CSS and JavaScript, and deferring the loading of non-critical resources until after the initial render. This can be achieved through techniques like code splitting, lazy loading, and inlining critical CSS. By streamlining the CPR, we ensure that the most important content is displayed as quickly as possible, leading to a better user experience and improved LCP scores. Understanding and addressing the CPR is fundamental to achieving optimal web performance, especially on mobile devices with limited resources and slower network connections.

Diagnosing the LCP Bottleneck: A Focus on JavaScript

To effectively tackle the LCP issue, a thorough diagnosis is essential. In this case, the primary suspect was JavaScript, specifically how it was being loaded and executed. JavaScript, while powerful, can be a performance bottleneck if not managed carefully. Render-blocking JavaScript, in particular, can delay the rendering of the page, directly impacting the LCP. This occurs when the browser encounters a <script> tag in the HTML and pauses parsing the HTML to download, parse, and execute the script. Only after the script has been executed does the browser resume parsing the rest of the HTML.

The investigation focused on identifying JavaScript files that were contributing to the delay. The team examined several key directories, including /scripts/, /blocks/, /libs/, /head.js, and /delayed.js, looking for opportunities to optimize JavaScript loading. This involved identifying render-blocking scripts, considering code splitting to break down large scripts into smaller, more manageable chunks, and strategically applying async or defer attributes to <script> tags. These attributes allow the browser to download scripts in the background without blocking HTML parsing (async) or to download scripts in the background and execute them after HTML parsing is complete (defer).

Another crucial aspect was identifying and removing unused JavaScript code and dependencies. Modern web applications often include a significant amount of JavaScript, some of which may not be necessary for the initial render. Eliminating this unnecessary code can reduce the amount of data the browser needs to download and process, leading to faster loading times. Additionally, lazy loading non-critical JavaScript was considered as a way to further optimize the loading process. This technique involves loading JavaScript only when it is needed, rather than all at once during the initial page load.

Analyzing JavaScript Files for Optimization

A comprehensive analysis of JavaScript files is crucial in identifying performance bottlenecks and optimizing the loading process. In this context, the team meticulously examined several key files and directories:

• /scripts/scripts.js: This file typically serves as the main script entry point for the website. It often contains global scripts and initialization logic. Analyzing this file helps identify any render-blocking scripts or opportunities for code splitting and optimization.
• /scripts/delayed.js: This file is specifically designed for scripts that can be loaded after the initial render. Moving scripts to this file can help reduce the load on the critical path and improve LCP.
• /blocks//*.js:** This directory contains JavaScript files associated with specific blocks or components on the website. Examining these files helps identify opportunities for lazy loading or optimizing the scripts for individual blocks.
• /head.js: This file often contains scripts that are loaded in the <head> of the HTML document. Scripts loaded in the <head> can be particularly impactful on performance, as they can block rendering. Analyzing these scripts is essential for identifying potential bottlenecks.

The analysis involved identifying render-blocking scripts, assessing the size and complexity of each script, and determining whether the script was essential for the initial render. The team also looked for opportunities to refactor code, remove unused dependencies, and implement lazy loading for non-critical scripts. By systematically analyzing these JavaScript files, the team was able to pinpoint the specific areas where optimization efforts would have the greatest impact on LCP.

Implementing the Solution: Deferring Non-Critical Scripts

Based on the diagnostic findings, the primary solution was to defer the loading of non-critical third-party scripts. This approach aligns with the principle of prioritizing critical resources for the initial render and delaying the loading of less essential assets. Specifically, the team identified auth0-spa-js.production.js as a script that could be moved to the delayed phase of the loading process. Auth0 is an authentication and authorization platform, and while its functionality is important, it's not necessarily required for the initial rendering of the page's main content.

Moving auth0-spa-js.production.js to load in the delayed phase meant that the browser would not need to download, parse, and execute this script before rendering the LCP element. This freed up valuable resources and allowed the browser to focus on rendering the critical content more quickly. The implementation involved modifying the HTML or JavaScript code to ensure that the script was loaded asynchronously or after the initial render.

The decision to defer this specific script was based on its size, its role in the website's functionality, and its impact on the critical path. By carefully analyzing the dependencies and loading behavior of various scripts, the team was able to make informed decisions about which scripts could be safely deferred without compromising the user experience. This targeted approach ensured that the optimization efforts were focused on the areas that would yield the greatest performance improvements.

Strategic Deferral of auth0-spa-js.production.js

The decision to defer the loading of auth0-spa-js.production.js was a strategic one, based on a careful assessment of its role and impact on the critical path. This script, related to authentication functionality, was not essential for the initial rendering of the main content. By deferring its loading, the team aimed to reduce the load on the critical path and allow the browser to prioritize rendering the LCP element.

The implementation of this deferral likely involved modifying the way the script was included in the HTML. Instead of a standard <script> tag that blocks rendering, the team may have used the async or defer attribute, or moved the script inclusion to a later stage in the loading process. The async attribute allows the script to be downloaded in parallel with HTML parsing, while the defer attribute ensures that the script is executed after the HTML parsing is complete.

Alternatively, the script could have been moved to the delayed.js file, which is specifically designed for scripts that can be loaded after the initial render. This approach ensures that the script does not block rendering and is loaded only when it is needed. The choice of implementation would depend on the specific requirements of the website and the desired loading behavior.

Results and Expected Outcomes: A 300-500ms LCP Reduction

The expected outcome of deferring non-critical scripts, particularly auth0-spa-js.production.js, was a significant reduction in LCP time, estimated to be in the range of 300-500ms. This improvement would translate to a noticeably faster perceived loading time for users, especially on mobile devices. A faster LCP not only enhances the user experience but also contributes to better SEO rankings, as Google considers CWV as a ranking factor.

The optimization efforts were specifically targeted at mobile performance, recognizing the importance of providing a smooth and fast experience for mobile users. Mobile devices often have limited resources and slower network connections compared to desktop computers, making them more sensitive to performance bottlenecks. By optimizing LCP on mobile, the team aimed to ensure that users could access and interact with the website quickly and efficiently, regardless of their device or network conditions.

In addition to the LCP improvement, the team also expected broader benefits from the optimization efforts. Reducing the load on the critical path can lead to faster overall page load times, improved First Contentful Paint (FCP), and a more responsive user interface. These improvements contribute to a better overall user experience and can lead to increased engagement and conversions.

Quantifiable LCP Improvement: The Target Range

The target LCP reduction of 300-500ms was a quantifiable goal that provided a clear benchmark for the optimization efforts. This range was based on an assessment of the existing LCP performance and the potential impact of the proposed optimizations. Achieving this reduction would represent a significant improvement in user experience, making the website feel faster and more responsive.

The actual LCP improvement would depend on several factors, including the specific implementation details, the network conditions, and the device capabilities. However, the target range provided a valuable guideline for the team and allowed them to track progress and measure the effectiveness of their efforts. Monitoring LCP before and after the optimization was crucial for validating the results and ensuring that the desired performance gains were achieved. The team likely used performance monitoring tools to track LCP and other CWV metrics, providing data-driven insights into the impact of their optimizations.

Ensuring Compatibility and Best Practices in AEM Edge Delivery

This project was implemented within the context of AEM Edge Delivery Services (EDS), which is a modern approach to building and delivering websites using Adobe Experience Manager (AEM). AEM EDS leverages a block-based architecture and a streamlined delivery pipeline to enable fast and efficient website performance. When optimizing performance in an AEM EDS environment, it's crucial to adhere to the platform's conventions and best practices to ensure compatibility and maintainability.

The team needed to ensure that their changes followed AEM EDS patterns and didn't break the block-based architecture. This meant carefully considering how the optimizations would interact with the existing components and templates, and avoiding any modifications that could negatively impact the overall structure and functionality of the website. Maintaining compatibility with the Franklin/Helix pipeline was also a key consideration. Franklin and Helix are the underlying technologies that power AEM EDS, and any changes needed to be aligned with their architecture and workflows.

Another important aspect was considering the impact of the changes on both the author and publish environments. Optimizations that improve performance on the publish environment should not negatively impact the authoring experience. This requires careful planning and testing to ensure that the changes are seamless and do not introduce any new issues. The team also needed to be mindful of the long-term maintainability of the codebase. Optimizations should be implemented in a way that is clear, concise, and easy to understand, making it easier for other developers to maintain and extend the website in the future.

AEM EDS Specific Considerations

Optimizing performance in AEM EDS requires specific considerations due to its unique architecture and delivery model. The block-based architecture, for example, means that optimizations should be focused on individual blocks or components, rather than the entire page. This allows for a more granular approach to performance tuning and ensures that changes are targeted and effective.

The Franklin/Helix pipeline also plays a crucial role in AEM EDS performance. This pipeline handles the transformation of content and code into a highly optimized format for delivery. Optimizations should be aligned with the pipeline's capabilities and limitations to ensure that they are fully effective. This may involve leveraging features like automatic code splitting, lazy loading, and image optimization, which are built into the Franklin/Helix pipeline.

The separation of author and publish environments in AEM EDS also requires careful consideration. Optimizations that are effective in the publish environment may not be appropriate for the author environment, and vice versa. For example, certain types of caching may be beneficial in the publish environment but detrimental in the author environment, where content is frequently updated. The team needed to carefully consider these factors when implementing their optimizations to ensure that they were beneficial across both environments.

Conclusion: A Holistic Approach to Web Performance Optimization

Optimizing web performance, particularly for mobile devices, requires a holistic approach that considers all aspects of the loading process. This case study highlights the importance of understanding the Critical Path Rendering (CPR) and identifying potential bottlenecks, such as render-blocking JavaScript. By strategically deferring non-critical scripts, the team was able to significantly reduce the Largest Contentful Paint (LCP) time, leading to a better user experience.

The key takeaways from this project include the importance of:

• Diagnosing performance issues: Thoroughly analyzing the website's loading behavior to identify the root cause of slow LCP.
• Prioritizing critical resources: Ensuring that the most important content is loaded first by optimizing the CPR.
• Deferring non-critical scripts: Delaying the loading of scripts that are not essential for the initial render.
• Considering the context of the platform: Adhering to the conventions and best practices of AEM Edge Delivery Services to ensure compatibility and maintainability.
• Measuring and validating results: Tracking LCP and other Core Web Vitals metrics to ensure that the optimizations are effective.

By adopting a comprehensive approach to web performance optimization, developers can create websites that are fast, responsive, and engaging, providing a superior user experience and achieving better SEO rankings. This project serves as a valuable example of how targeted optimizations can lead to significant performance improvements, ultimately benefiting both users and the website's overall success.