In this article, you’ll learn the impact of JS execution on performance, how you can measure and identify the JS issues, and how you can reduce the JS execution time on your WordPress site, both manually and with some plugins.
What’s the Impact of JS Execution Time on Performance?
You can see how the JS execution time affects performance by looking at the First Input Delay metric and the Lighthouse performance score.
Let’s see in more detail what it means.
How to Measure JS Execution Time and Identify the Main JS Issues
Reducing JS execution time means reducing the time spent by the browser to parse, compile and execute JS once downloaded.
Parsing, compiling, and executing are the three steps you should manage to reduce the JS execution time.
Every time the JS execution is longer than 2 seconds, Lighthouse will report it as an issue to solve.
You can identify and measure JS execution time via PageSpeed Insights and GTmetrix.
Measuring JS Execution Time via Pagespeed Insights
If your URL has any JS Execution time issues, Google PageSpeed Insights will show a warning in the Diagnostics section:
As you can see, PSI shows the JS execution time by each step:
- Script evaluation (it’s the compiling stage)
- Script parse (it’s the parsing stage)
- Total CPU Time (it’s the memory cost, which can freeze up your page completely).
As mentioned above, the audit considers each JS script that has an execution time longer than 2 seconds.
Measuring JS Execution Time via GTmetrix
When testing your URL’s performance, GTmetrix shows you a very similar audit.
Go to the Structure tab, and look for the status of the JS execution time. You’ll then get the information about each JS stage.
In the example below, the JS execution time is above 2 seconds.
How to Reduce JS Execution Time (Manually and With Plugins)
There are three main ways to reduce JS execution time:
- Delay JS files and remove unused JS
- Defer JS
- Minify JS.
Let’s see how you can implement each of these actions and address the PageSpeed Insights recommendation.
1. Delay JS and Remove Unused JS
You should delay JS files so that none of them will be loaded until user interaction. It means that as long as no user clicks on a button or scrolls a page’s content, the browser will not execute any JS scripts.
You should also remove unused JS files. In other words, all the JS scripts that are useless or not included in the above-the-page content should be loaded only after the page’s rendering. It’s a way to let the browser render only the essential content without getting stuck with not-needed JS files.
Several plugins can help you delay JS and remove unused JS:
2. Defer JS
Another effective way to deal with the JS execution time is to defer JS files.
You have different ways to defer JS files, both manually and with plugins.
If you want to implement the defer attribute yourself, you should first identify the JS scripts to defer and then add the defer attribute.
Here’s an example of the defer attribute:
<script defer src=”/example-js-script”></script>
If you prefer using a plugin, you can use Asset CleanUp.
In the File optimization tab, you’ll find the option to Load JS deferred and exclude any specific JS files from being deferred, too. In a few clicks, you’ll take care of a crucial web performance optimization technique!
3. Minify JS
The third option to tackle JS issues is through minification. Minification removes the line breaks, white spaces, and comments included in the code. As a result, the code is lighter, and the file size is faster.
Since minification is a time-consuming activity, and there’s always the risk of missing out on something, we recommend using a plugin.
Once again, WP Rocket offers you an easy way to minify JS files. Go to the File Optimization tab and enable the Minify JS files option. Easy and fast as that!
Other WordPress Recommendations to Reduce JS Execution Time
As a WordPress user, there are other useful recommendations to keep in mind.
- Use a static image instead of a slider (especially if the slide is at the top of the page)
- Avoid overusing widgets such as social network (e.g., the social widgets in your sidebar)
- Avoid overusing 3rd party resources (e.g., ads, trackers, personalization layers, A/B testing).
Start Reducing JS Execution Time Today
Reducing JS Execution time is a crucial step in improving the FID score and boosting your PageSpeed Insights performance score. Overall, it’s an essential way to provide your users with an excellent user experience.
Thanks to WP Rocket, you can manage all the main JS issues in just a few clicks. You’ll save time and effort!
Not a WP Rocket customer yet? Save yourself time and let WP Rocket do the job for you. WP Rocket is the easiest way to improve your PageSpeed Insights score.
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