Optimizing Page Speed: Reducing Render-Blocking Resources to Meet User and Google Expectations
Despite notable changes in the organic search environment throughout the year, the speed and functionality of web pages continue to hold great importance. Users still demand quick and seamless online experiences, with 83% expecting websites to load in three seconds or less. Google sets even higher standards, requiring a Largest Contentful Paint (LCP) metric of under 2.5 seconds to be regarded as "Good."
Unfortunately, reality often falls short of both Google’s and users’ expectations, as the average website takes 8.6 seconds to load on mobile devices. However, there is a positive trend: this figure has decreased by 7 seconds since 2018 when the average mobile page load time was 15 seconds. Page speed is not just about total load time but also about the user experience during those 3 (or 8.6) seconds. It’s crucial to assess how efficiently pages render by optimizing the critical rendering path to achieve the first paint as quickly as possible. This involves minimizing the time users spend staring at a blank screen and getting visual content displayed as soon as possible.
Understanding the Critical Rendering Path
The critical rendering path refers to the steps a browser takes to render a page, transforming HTML, CSS, and JavaScript into visible pixels on the screen. Essentially, the browser must request, receive, and interpret all HTML and CSS files before beginning to render any visual content. This process results in users seeing a white screen until completion.
Optimizing the Critical Rendering Path
To optimize the critical rendering path, prioritize resources for rendering meaningful, above-the-fold content. Identify and deprioritize render-blocking resources not necessary for loading above-the-fold content that may slow down the page rendering. Start by inventorying critical resources and seek opportunities to:
- Reduce the number of critical resources by deferring render-blocking resources.
- Shorten the critical path by prioritizing above-the-fold content and downloading all critical assets early.
- Minimize the number of critical bytes by reducing the file size of remaining critical resources.
Render-Blocking Resources
Render-blocking resources include webpage elements that must be loaded and processed before the browser can render content. These generally include CSS and JavaScript files.
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Render-Blocking CSS: CSS is inherently render-blocking and must be fully loaded before rendering to ensure styled content. Without CSS, pages would be unusable and need repainting. The time taken for this varies based on the size and quantity of CSS resources.
- Render-Blocking JavaScript: Unlike CSS, the browser doesn’t need to download and parse all JavaScript resources to render the page. Yet, encountering JavaScript before page rendering requires execution first, delaying rendering. JavaScript has the potential to alter HTML and CSS elements, so the browser pauses rendering to manage possible changes.
Impact on Core Web Vitals
Core Web Vitals (CWV) are metrics set by Google to gauge a user’s page experience, focusing on loading, interactivity, and visual stability:
- Largest Contentful Paint (LCP): Evaluates loading performance by measuring the time for the largest visible content to load.
- Interaction to Next Paint (INP): Evaluates responsiveness by measuring interaction time until browser response.
- Cumulative Layout Shift (CLS): Measures visual stability by tracking unexpected layout shifts.
Optimizing the critical rendering path mainly affects LCP by determining how quickly significant content loads. Proper optimization reduces LCP time and can also enhance INP and CLS, allowing quicker interactions and more predictable resource loading.
Identifying Render-Blocking Resources
To identify render-blocking resources, utilize tools like PageSpeed Insights, Lighthouse, and WebPageTest.org. These tools flag resources that hinder optimal page rendering, such as JavaScript and CSS files without defer attributes. Use waterfall views from tools like WebPageTest.org to visually analyze resource loads and identify non-essential pre-render resources.
Testing Resource Impact on Above-The-Fold Content
Evaluate whether deferring identified resources affects above-the-fold content by:
- Using a browser incognito window.
- Accessing the “Request blocking” tab in DevTools.
- Blocking identified resources.
- Assessing content appearance after blocking.
Reducing Render-Blocking Resources and Prioritizing Critical Resources
Here’s how to approach it:
- Place JavaScript at the Bottom: This best practice lowers blocking potential.
- Async/Defer Attribute: Use these attributes to load scripts asynchronously, minimizing parsing delays.
- Custom Solutions: Onload functions can defer resource loading, keeping critical rendering unhindered.
- CSS Media Queries: Control when CSS parses, saving time where resources aren’t critical.
- Prioritize Critical Resources: Ensure significant items load first using resource hints and other methods, like inlining small critical bits.
Optimizing the critical rendering path can significantly improve page load times and user experiences. Explore different strategies to defer, reduce, and prioritize resources for a performance-focused web experience.
