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How to delay JavaScript until user interaction in WordPress

While defer JavaScript pushes the script to the bottom of the page load, you can instead delay JavaScript based on user interaction. This can be a great way to significantly speed up the paint of the page for Google Lighthouse when something isn’t needed right away. Especially heavy third-party scripts like Google Adsense, Google Analytics, conversion pixels (FB, Google Ads), etc.

Delaying JavaScript has a big impact on Largest Contentful Paint (LCP) and can help you fix the “Reduce unused JavaScript” warning in Google PageSpeed Insights. It will decrease your Total Blocking Time (TBT) and help fix the “Reduce the impact of third-party code” and “Reduce JavaScript execution time” warnings.

Reduce unused JavaScript warning in PageSpeed Insights
Reduce unused JavaScript warning in PageSpeed Insights

It also works great for things like cart fragments which is a common WooCommerce performance issue due to the fact that the AJAX request can’t be cached. You can also use it with our local analytics feature and Instant Page.

Delay JavaScript

Follow the steps below to delay JavaScript with Perfmatters. This should work with a script tag with either the type='javascript' or no type.

Step 1

Click into the Perfmatters plugin settings.

Perfmatters plugin settings
Perfmatters plugin settings

Step 2

Click on the “Assets” tab.

Perfmatters Assets
Perfmatters Assets

Step 3

Scroll down and under the “JavaScript” section, toggle on “Delay JavaScript.”

Delay JavaScript
Delay JavaScript

Step 4

Then proceed to choose your Delay behavior.

Delay behavior

There are two different methods you can choose from to delay JavaScript:

  1. Only delay specified scripts
  2. Delay all scripts
Perfmatters JavaScript delay behavior
Perfmatters JavaScript delay behavior

1. Only delay specified scripts

Delay specific JavaScript files by adding the source URL (example.js), or delay an inline script by adding a unique string from that script. Format: one per line

Delay specified JavaScript with Perfmatters
Delay specified JavaScript with Perfmatters

2. Delay all scripts

The delay all scripts feature will attempt to delay all JavaScript, including inline. This is by far the most aggressive option. You can then exclude scripts if needed.

Excluded from delay

Exclude specific JavaScript files from delay by adding the source URL (example.js), or exclude an inline script by adding a unique string from that script. Format: one per line

Excluding JavaScript from delay
Excluding JavaScript from delay

Delay timeout

With both delay behavior options, you can toggle on a timeout that will automatically load scripts after 10 seconds if no user interaction has been detected. This is optional and off by default.

JavaScript delay timeout
JavaScript delay timeout

Here are a few examples of why you might want to enable a timeout:

  1. Help improve data accuracy of tracking scripts, especially if you have a goal associated with a pageview, but maybe no user interaction. You want the goal to trigger still.
  2. You have a Google AdSense blog, and a user is opening up multiple pages in new tabs in the background. You still want the ads to fire off.

If you want to set a different delay timeout value, you can use the perfmatters_delay_js_timeout filter.

Delay inline JavaScript

To delay inline JavaScript, enter a partial string that is within the <script type="text/javascript"> </script> tag.

For example, say we have this inline JavaScript.

<script type="text/javascript">var lazyLoadInstance=new LazyLoad({elements_selector:"[loading=lazy],.perfmatters-lazy",thresholds:"200% 0px"});</script>

We can add lazyLoadInstance to the “Delay JavaScript” box and it will delay it. You can choose any string within the script tags, just make sure it’s unique.

For some JavaScript, they might have a source file as well as an inline component. In this case, you will need to delay both for it to work properly.

Examples of scripts you can delay

Below are some examples of common scripts we delay on WordPress sites.

Delay Google Analytics

To delay Google Analytics, add the following to the “Delay JavaScript” box.

ga( '
ga('
google-analytics.com/analytics.js

If you’re using our local analytics, you can also delay the scripts.

Local Perfmatters analytics.js

To delay the local Perfmatters analytics.js script, add the following to the “Delay JavaScript” box.

analytics.js

Local Perfmatters gtag.js v4

To delay the local Perfmatters gtag.js v4 script, add the following to the “Delay JavaScript” box.

gtagv4.js

Local Perfmatters Minimal

To delay the local Perfmatters analytics-minimal.js script, add the following to the “Delay JavaScript” box.

analytics-minimal.js

Delay Google Tag Manager

To delay Google Tag Manager, add the following to the “Delay JavaScript” box.

/gtm.js
/gtag/js
gtag(
/gtm-

Google Adsense

To delay Google AdSense, add the following to the “Delay JavaScript” box.

adsbygoogle.js

If you’re using a plugin like Ad Inserter, along with HTML insertion blocks, you’ll want to delay both the script and the inline portion.

adsbygoogle.js
ai_insert_code

Delay Facebook Pixel

To delay the Facebook Pixel, add the following to the “Delay JavaScript” box.

fbevents.js
fbq(
/busting/facebook-tracking/

Above are just a few of the many examples of scripts you can disable. If you are having any trouble delaying a certain script, feel free to contact us.

How to exclude delayed scripts

If you’re using the Delay All JavaScript feature, you can add scripts to the “Excluded from Delay” box.

How to exclude a page/post from delay

You can exclude JavaScript delay on an individual post, page, or custom post type. In the editor, on the right-hand side uncheck “Delay JavaScript.” This will exclude the entire page from delay. This can be useful for say a checkout or contact us page that might have more issues than the rest of the site. Note: You must be an administrator to see the meta options.

JS delay exclusion on individual page
JS delay exclusion on individual page

Common JS delay exclusions

When using the Delay all scripts option in Perfmatters, you might need to add an exclusion or two. The number is typically based on the complexity of the site. For example, a site using a heavy page builder like Elementor will most likely need more exclusions than a lightweight site using GeneratePress.

Below are some common exclusions we typically see. We’ll make sure to keep adding to this list as we improve the feature.

Astra mobile menu

jquery.min.js
astra

Avada sticky header

avada-header.js
modernizr.js
jquery.easing.js
avadaHeaderVars

Divi mobile menu

jquery.min.js
/themes/Divi/js/scripts.min.js
et_pb_custom

Elementor mobile menu

This might vary slightly based on the elements and templates you’re using in Elementor.

jquery.min.js
jquery.smartmenus.min.js
webpack.runtime.min.js
webpack-pro.runtime.min.js
elements-handlers.min.js
frontend.min.js
frontend-modules.min.js
elements-handlers.min.js
elementorFrontendConfig
ElementorProFrontendConfig
imagesloaded.min.js
webpack-pro.runtime.min.js
webpack.runtime.min.js
elements-handlers.min.js
jquery.smartmenus.min.js

GeneratePress mobile menu

/generatepress/assets/js/menu.min.js
generatepressMenu

GeneratePress mobile popup menu

/generatepress/assets/js/menu.min.js
generatepressMenu
offside.min.js
offSide

OceanWP mobile menu

drop-down-mobile-menu.min.js
oceanwpLocalize
jquery.min.js
flexslider
single-product.min.js
slick
functions.min.js
waypoint

How to filter delayed scripts

You can use the perfmatters_delayed_scripts filter to manipulate your delayed scripts. For example, you could exclude a script from being delayed on a desktop device.

add_filter('perfmatters_delayed_scripts', function($scripts) {
     if(($key = array_search('name_of_your_script', $scripts)) !== false) {
         if(!wp_is_mobile()) {
             unset($scripts[$key]);
         }
     }
     return $scripts;
 });

You can use the perfmatters_delay_js filter to turn your delayed scripts on or off entirely. For example, you could turn off delay only on pages.

add_filter('perfmatters_delay_js', function($delay) {
      if(is_singular('page')) {
        return false;
    }
    return $delay;
});

Delay versus Defer

Defer will push the JavaScript to the bottom of the page load. This helps resolve render-blocking issues. However, the JavaScript still has to load when the page is rendered. In other words, it will still play an impact on your Core Web Vital scores when it comes to JavaScript execution time, transfer sizes, etc.

Delay removes the JavaScript from the waterfall/page load altogether. If you run a speed test, it will appear as if those scripts don’t exist. They do, but instead of loading on page load, they load on user interaction (moving mouse, scrolling, timer, etc.) This helps improve your Core Web Vital scores and improves the first contentful paint (FCP).

Regarding compatibility, defer is a practice that has been around for a long time. It’s usually safe to defer everything, which is what the Defer JavaScript feature in Perfmatters does. And then we have the exclusions box for cases where a script might need to be excluded due to load order.

Delay is a little trickier, as you can’t just delay every JavaScript file. Due to loading on interaction, it will only work on specific scripts. This can be a little trial and error, but the benefit of delay is that it’s very powerful! It has a massive impact on each script you use it on.

The good news is, you can use both delay and defer simultaneously. If both are applied to the same JavaScript file, delay will always take precedence, and defer is essentially then a fallback. If you delay all of your JavaScript, defer can still be useful as you might have delay exclusions, which would then use defer.

Troubleshooting delay

There are a few things to keep in mind when using delay, especially how it impacts certain scripts.

Turn off combine/compilers

For Delay JavaScript to work properly in Perfmatters, you need to turn off any feature from third-party plugins that combine your JavaScript files. Combining JavaScript is a deprecated optimization technique since HTTP/2 (now HTTP/3) anyways. It’s now faster to load individual files in parallel.

  • If you’re using Autoptimize, LiteSpeed, WP Rocket, etc., turn off JavaScript combine.
  • If you’re using Avada, turn off the JS Compiler feature.

Google Analytics

For example, with Google Analytics, you might see slightly lower stats if they’re delayed. That’s because the script is now loading on user interaction. However, this isn’t necessarily a bad thing. Google doesn’t filter out all of the bots or spam, so if you delay Google Analytics, along with the delay timeout feature, it might improve your data as it now narrows it down to actual users.

Bounce rate measurements might also fluctuate if you delay Google Analytics.

How to fix double-click on mobile menu

If your mobile menu isn’t working on the first click, you probably need to add an exclusion for the JavaScript that is powering it. See our common exclusions.

How to fix double-click in iOS on hyperlinks

There is a known bug on iOS that sometimes occurs when using the Delay JS feature. This bug causes links to not work until they are clicked twice. We hope that iOS will eventually change a few things so that this won’t happen anymore. However, until then, there is a temporary workaround using the FastClick library. Follow the steps below.

Step 1

Add the following code (source) to the header of your WordPress site. You can use the Add Header Code box in Perfmatters.


<script src="https://cdnjs.cloudflare.com/ajax/libs/fastclick/0.6.0/fastclick.min.js" integrity="sha512-oljyd1wg75alHReTpDvNIQ4Yj1wZwGxxZhJhId3vr2dKY+26/r/wmMrImwDgin03+7wxyhX+adOQB/2BTvO5tQ==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
<script>
if ('addEventListener' in document) {
	document.addEventListener('DOMContentLoaded', function() {
		FastClick.attach(document.body);
	}, false);
}
</script>

Step 2

Exclude the following from Delay JavaScript execution:

fastclick

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