When you load a webpage, your browser reads through the HTML from top to bottom, building the page as it goes. The moment it encounters a CSS stylesheet or a JavaScript file referenced in the <head>, it stops. It cannot continue rendering the page until that file has been downloaded, parsed, and processed. Those files are render-blocking resources, and they are one of the most common reasons a page feels slow even when the underlying server response is fast.
How to Identify Render-Blocking Resources
Google PageSpeed Insights is the simplest starting point. Run your URL through it (pagespeed.web.dev) and look for the "Eliminate render-blocking resources" opportunity in the results. It will list the specific files causing the delay and estimate how much time eliminating them would save.
The browser's own developer tools - available in Chrome and Firefox by pressing F12 - give you a more detailed waterfall view. The Network tab shows each file loaded in sequence, with bars indicating when each request started and how long it took. Files that delay the start of page rendering are visually obvious as blockers in this timeline.
Fixing JavaScript Render-Blocking
JavaScript is usually the larger problem. A script referenced in the <head> without any special attribute causes the browser to stop, fetch the script, execute it, and only then continue. There are two attributes that change this behaviour.
defer tells the browser to download the script in the background while HTML parsing continues, then execute it after the HTML is fully parsed. The execution order is preserved, which matters for scripts that depend on each other. This is the right choice for most WordPress theme and plugin scripts.
async downloads the script in the background and executes it the moment it finishes downloading, regardless of where the HTML parser is. Execution order is not guaranteed. Use this only for independent scripts - analytics tags and similar tools are good candidates.
Moving scripts to just before the closing </body> tag achieves a similar effect: the browser renders the visible page first, then loads the script. Many WordPress plugins already output scripts in the footer, but some do not, and this is configurable.
Fixing CSS Render-Blocking
CSS is trickier because browsers intentionally block rendering until stylesheets load - otherwise they would show unstyled content to the user, which is generally worse than a slight delay. The standard approach is to identify the CSS rules needed to render what is visible above the fold (called critical CSS), inline those styles directly into the HTML, and load the rest of the stylesheet asynchronously.
Doing this manually is tedious and requires updating whenever your above-the-fold content changes. This is why most sites handle it through a performance plugin rather than manually.
Using a Caching Plugin to Automate This
For WordPress sites, the most practical path is a caching and performance plugin that handles render-blocking issues automatically.
WP Rocket is the most capable option in this category. It handles script deferral, async loading, critical CSS generation, and stylesheet optimisation through a straightforward settings panel. The time-to-first-render improvements from enabling these options are often measurable on the first test after configuration.
LiteSpeed Cache offers similar functionality and is particularly effective on hosting environments running the LiteSpeed web server. dotCanada's hosting infrastructure supports LiteSpeed, which means you can take full advantage of its page cache and optimisation features.
Autoptimize is a free option that handles CSS and JS minification and concatenation, and can defer scripts. It requires a bit more configuration to get right but is capable when tuned properly.
Prioritising What to Fix
Not every render-blocking resource is equally impactful. Focus first on the files PageSpeed Insights flags as contributing the most delay. Often two or three files account for the majority of the problem.
Scripts from third-party services - live chat widgets, social media embeds, advertising tags - are frequent culprits and often harder to address because you cannot change the script itself. Loading these scripts asynchronously is essential. Some may also offer a tag manager approach where you can control their loading priority.
Render-blocking resources are one of the clearest opportunities to improve Core Web Vitals scores - particularly Largest Contentful Paint (LCP) and First Contentful Paint (FCP). Fixing them is among the highest-leverage performance improvements available to most WordPress sites.
