Category: Forensic Analysis
Reading Time: 6 Minutes
Audience: Developers, Technical Founders, Growth Marketers
Klaviyo is non-negotiable for retention marketing. But from an engineering perspective, it is often the heaviest script in the Shopify ecosystem.
If you run a standard "App Embed" installation, klaviyo.js
injects itself into the <head>
of your document. This forces the browser to download, parse, and execute a massive library before it
even paints your Largest Contentful Paint (LCP).
In my audits, I consistently see Klaviyo responsible for 300KB+ of render-blocking JavaScript on mobile devices.
Figure 1: Forensic analysis showing klaviyo.js
injecting a 313 KiB payload. Even if execution is deferred, this massive library competes for
critical bandwidth during the initial load.
You do not need to choose between email revenue and site speed. You just need to change when the library is allowed to execute.
The Forensic Diagnosis
Open your Chrome DevTools Network Tab. Filter by "JS" and reload your page.
If you see klaviyo.js
loading in the first 500ms—fighting for bandwidth alongside your Hero Image and your main CSS—you have a
structural latency problem. This is Render Blocking.
The default integration treats the script as "Critical," but it isn't.
- Forms: A user is not going to sign up for your newsletter in the first 3 seconds of landing on the page.
- Tracking: I can capture the "Active on Site" metric without blocking the initial render.
The "Ghost" Double-Load
Before applying the fix, check your theme.liquid.
I often find that merchants have enabled the "App Embed" in the Theme Editor (Shopify 2.0) but forgot to remove the hardcoded script tags from a previous installation years ago.
If you see both, you are loading the library twice. Delete the hardcoded tag immediately.
The Fix: Script Deferment & Interaction Listeners
The solution is to "Defer" the loading of Klaviyo until the browser is idle or the user interacts with the page. I want the Main Thread to be purely focused on rendering the UI first.
Step 1: Disable the App Embed
Go to your Theme Editor > App Embeds and toggle Klaviyo OFF. I am going to handle the injection manually.
Step 2: Implement the Deferment Logic
Add the following script to your theme.liquid
(or a dedicated snippet), just before the closing </body>
tag.
This script waits for a "User Interaction" (scroll, mousemove, or touch) before injecting Klaviyo. This guarantees that Lighthouse sees an idle Main Thread during the critical initial load.
<script>
(function () {
const PUBLIC_API_KEY = 'YOUR_SIX_CHAR_PUBLIC_KEY'; // <--- Update this
let klaviyoLoaded = false;
function loadKlaviyo() {
if (klaviyoLoaded) return;
klaviyoLoaded = true;
console.log('FosterUI: Injecting Klaviyo via Interaction Listener');
const script = document.createElement('script');
script.type = 'text/javascript';
script.async = true;
script.src = `//static.klaviyo.com/onsite/js/klaviyo.js?company_id=${PUBLIC_API_KEY}`;
const firstScript = document.getElementsByTagName('script')[0];
firstScript.parentNode.insertBefore(script, firstScript);
}
// The "Interaction" Triggers
const userInteractions = ['scroll', 'mousemove', 'touchstart', 'keydown'];
const triggerLoad = () => {
loadKlaviyo();
userInteractions.forEach(e => window.removeEventListener(e, triggerLoad));
};
userInteractions.forEach(event => {
window.addEventListener(event, triggerLoad, { passive: true });
});
// Safety Net: Load after 4 seconds if no interaction occurs (preserves attribution)
setTimeout(loadKlaviyo, 4000);
})();
</script>Critical Safety Note: Prevent Layout Shifts
If you use embedded Klaviyo forms (like a footer newsletter signup), this deferment will cause the form to 'pop in' later. To prevent Layout Shift (CLS), you must set a min-height in your CSS for the form container div so the space is reserved before the script loads.
Step 3: Verify the Waterfall
Reload your Network Tab. You should now see klaviyo.js
loading after the green LCP line. Your TBT score should drop significantly, and your "Time to
Interactive" will improve.
The Result
This is not a "hack." This is resource prioritization.
I am telling the browser: "Paint the product image first. Load the marketing tools second."
Klaviyo is just one of 15 scripts likely fighting for your Main Thread. I can trace your entire waterfall to find out exactly what else is killing your mobile performance.
Note for Marketers: The 4-second fallback ensures that UTMs and _kx
attribution parameters are still captured, even if the user doesn't scroll immediately. Data integrity
is preserved.
Get the Forensic Audit below.