Critical Rendering Path – Reduce Bounce Rates with Optimum Website Performance

Website rendering is the process of converting code into displayable pixels in your browser. Unfortunately, slow website rendering is one of the biggest reasons for high bounce rates. Therefore, you should optimize your website's rendering performance to grow traffic and reduce churn.

So, we bring the best tips to speed up your website's first render in our guide below. But, first, a little jargon is in order.

What is the Critical Rendering Path?

In plain speak, the Critical Rendering Path (CRP) is the process of converting the code to the displayed UI on your screen. CRP occurs in several stages:

  • Request the web page: You first request the desired web page by sending an HTTP request to the URL.
  • Start Parsing the response: Once the browser receives the code in response, it starts parsing it.
  • JavaScript Dependency: If the browser finds a JavaScript file as it parses the response, it first needs to download the file before it can resume parsing because the JavaScript affects all other page components. So, we call JavaScript parser blocking.
  • CSS Dependency: If a CSS stylesheet is in the code, the browser will parse it fully first before rendering as the CSS affects the display output. Thus, we call CSS render-blocking.
  • Build the DOM, CSSOM and the Render Tree: Once the parsing finishes, the browser compiles all the data into the Document Object Model (DOM ) and the Cascading Style Sheets Object Model (CSSOM). It then combines both into the Render Tree, which contains all the information to display the UI.
  • Rendering the Render Tree: Finally, the browser converts the Render Tree into the Layout and renders this Layout as the displayed UI. The final stage of actually displaying the UI is called the Paint stage.

Optimization Factors – What Decides Rendering Speed?

The breakdown of the browser rendering process gives us an insight into the factors that impact page rendering speed.

  • The Volume of Data to Download/Upload
  • The Number and Size of Dependencies – Especially Render Blocking (CSS) Resources and Parsing Block (JavaScript) Resources
  • The Time Duration of the CRP Process

Optimization Metrics – What to Track?

We must drift to one final aside before diving into the best rendering optimization strategies. First, we need to convert our goal of “rendering optimization” into concrete metrics.

So, we will track the following three key parameters to measure how well we optimize our page rendering.

  • First Paint: In rendering terms, “Paint” refers to the actual render output. So First Paint is the time it takes to display the first pixel on the screen when a page loads.
  • First Contentful Paint: First Contentful Paint is when the first "content" pixels display. It is the point at which the first DOM element starts showing up, and the user feels that the content is coming through. Note First Paint could be non-content visual cues like background color changes, but First Contentful Paint has to be some DOM element.
  • First Meaningful Paint: First Meaningful Paint is when the first useful information in line with the user's search intent shows up. The choice of a meaningful DOM element is subjective, but headings and relevant images are generally used to track this metric.

You can monitor these rendering performance metrics in your browser’s developer tools under the “Performance” tab.

Optimize Critical Render Path – Best Strategies for Superior Render Performance

Let's jump right into the best strategies to get top-notch rendering performance for your website without any more delay.

Reduce the Volume of Data to Download/Upload

First up, we will reduce the amount of download data and any further request data to upload.

Remove Unused Code Snippets: The first step is to refactor our code to make it efficient and small in size for fast download. So, we remove all our code parts which are never used. Some common examples of such code bytes could be:

  • JavaScript functions that are never called or are unreachable
  • CSS style selectors which do not match any DOM element
  • Permanently hidden HTML elements

DRY (Do Not Repeat): DRY down your code by removing redundancies and duplicate code. Watch out for stuff such as:

  • Repeated logic – Extract them into functions
  • Needless branching that can be optimized
  • Use loops for any multiple hand-coded calculations

Minification: Use a good auto-minification tool to remove non-code characters like white spaces and comments to convert your source code file into a running-text file for fast download.

Compress the Minified File for Faster Transfer

Most browsers support common compression file formats. For example, GZIP is a good algorithm you can use to compress the minified file.

Optimize for caching: Finally, optimizing your files for caching will reduce load times in subsequent page visits after the first one. Use the following two tips to use caching:

  • Ensure your Content Delivery Network (CDN) supports caching and the necessary settings.
  • It is best to proactively update your files by putting their fingerprints in file URLs. This helps invalidate local caches and helps visitors get the best site performance with the latest resources.

Optimize Images: Finally, compress your images and use proper image sizes for faster loads.

Cut Down on the Number of Dependencies to Download (JavaScript and CSS Files)

We reduce the number of JavaScript and CSS files that directly affect the render process. Such dependencies are called "critical resources". We can leave those files that do not impact the rendering process.

  • Reduce the Number of CSS Files to Download
  • Use the “media” attribute to only download the stylesheets specific to the device. For example, we can match our device by screen size and device type by setting the proper values in the "media" attribute.
  • Use "Inline" styles. We can choose wisely and embed some inline style for quick styling to speed up the render. Inline-styles are local and do not require any time-consuming network calls.
  • Optimizing JavaScript Files
  • Critical JavaScript resources are parse blockers because they affect the DOM and the CSSOM. So, we must release any JavaScript file that does not affect these.
  • Use the “async” attribute in the “script” tag to download the JavaScript file in the background asynchronously.
  • Use the “defer” attribute to load the JavaScript after the page load finishes.

Shorten the Critical Rendering Path Duration

All the above steps reduce the length of the critical rendering path. You can also rearrange code to optimize the critical rendering path length further. For example, you can structure your code to render the ATF (Above the fold) content first up and block other parsing and downloading for this duration. Once the ATF content engages the reader, you can resume the rest of the parsing.


Good rendering performance can reduce bounce rates and improve engagement and click-through metrics. So, we need to optimize the Critical rendering Path (CRP) to get the best load times. The tried-and-tested methods in our guide above will help you achieve industry-best results for rendering performance metrics.


  • How long should a website take to load?

The ideal time to load a website should be less than one second. But, First Paint times matter more as you need to hook the user and keep him engaged while the rest of the site loads in the background.

  • How to track website rendering performance?

You should use the “Performance” tab in your browser’s Developer Tools to track and optimize your site’s rendering performance.

  • How can I improve my website's rendering performance?

You need to optimize your web page’s Critical rendering Path to get fast load times. Please use the methods in our guide above to improve rendering performance.

Rachin Kapoor

Rachin Kapoor

Montreal, QC, Canada