INP Reductions in Shopify and BigCommerce: Script Budgets and Event Handlers

The key to making your make your online store visible and boosting its revenue lies in making lasting changes to its Core Web Vitals (CWV), which include its Largest Contentful Paint (LCP), Cumulative Layout Shift (CLS), and Interaction to Next Paint (INP). 

INP, in particular, measures how long a browser takes to respond to a user’s actions (like clicks and taps) and visually update the interface. Google considers an INP under 200ms to be “good;” anything slower introduces perceived lag, leading to higher bounce rates and lower engagement. 

On both Shopify and BigCommerce, ecommerce CWV improvements that will help you improve INP come down to managing JavaScript budgets and minimizing event handler overhead. Below is everything you need to know. 

The Basics of INP

Where LCP focuses on loading and CLS measures stability, a page’s INP evaluates its ongoing responsiveness, capturing its full input lifecycle:

• Input Delay: Time waiting for the main thread to become available 
• Processing Delay: Time taken to execute the event callback 
• Presentation Delay: Time required to paint the next frame on screen 

Anything that ties up the browser’s main thread, such as complex scripts or heavy analytics tags, can degrade INP performance. But once you know what moves the needle, you can make impactful ecommerce CWV improvements that benefit your bottom line. 

Shopify Constraints and Optimization Opportunities

As you prepare to improve INP metrics across your site, you need to know what hurdles stand in your way. Major constraints you’ll face on Shopify, specifically, include the following: 

Platform Architecture Considerations

Shopify’s Liquid templating system renders pages server-side, but user interactions, such as quick-add carts and predictive searches, rely on client-side JavaScript (JS). And because these scripts execute sequentially, large bundles or unoptimized app code can delay the input response. 

With that in mind, prioritize reducing render-blocking resources and deferring all non-essential scripts through the theme editor or theme.liquid settings. Inline only what’s critical to a user’s initial interaction, and defer everything else. 

Script Budgets and App Management  

A JS budget defines how much script execution a page can afford before its responsiveness declines. The problem with Shopify is that its app ecosystem often bloats such a budget. 

Perform a script audit using Chrome DevTools or Lighthouse to identify which apps consume the most main-thread time. Disable any unused App Embeds within the Admin panel, and consolidate overlapping functionality. 

Navigation and Tag Hierarchies

Shopify lacks a native sub-collection hierarchy, and in response, stores simulate subcategories using tags and nested menus. While flexible, that adds significant Document Object Model (DOM) weight and creates redundancies. 

Here are a few ways to mitigate these effects:

• Load navigation drawers only after user interaction 
• Limit mega menus to essential categories
• Cache frequently used menu JSON responses in local storage to prevent repeated fetches 

While you need sub-collections, you must also ensure that your tags and nested menus do not create crawl traps that waste your crawl budget. Otherwise, you will have a tough time maintaining a strong ranking, even if other elements of your site are adequately optimized. 

BigCommerce INP Challenges

BigCommerce has its own share of INP hurdles that you’ll need to overcome, such as:

Theme and Template Behavior

BigCommerce’s Stencil framework includes numerous built-in event listeners that track filters, pagination, and cart updates. Layering custom scripts on top can, over time, cause multiple listeners to fire on a single input. Conduct an audit of your theme’s JavaScript modules, paying attention to global events like scroll, input, and resize. If multiple scripts react to the same trigger, consolidate or throttle them. 

Removing certain dependencies and relying on native browser APIs improves execution efficiency and frees up the main thread, resulting in an improved INP without visual regressions that negatively impact consumers. 

Canonical and Faceted Navigation

Faceted category URLs in BigCommerce can generate numerous indexable variants that impact crawl load and indirectly impact your site’s performance through repeated rendering.

Enforce canonical control via your category templates and exclude filter parameters from both robots.txt and your sitemap. That will ensure Google crawls a single canonical version, reducing crawl depth and resource consumption. 

Here are some targets to aim for:

• A crawl depth of three or less
• Zero indexable parameter pages
• Canonicalized category URLs across all filters

The goal is to create a more crawlable site while also allowing consumers to reach the pages they are looking for in just a few (i.e., under three) clicks. 

Making Ecommerce CWV Improvements with JavaScript Budgeting and Event Handler Discipline 

JavaScript and event handler discipline are the keys to improving INP. An effective script budget will help you be more disciplined when adopting new features. Target thresholds include:

• JS transfer size of less than or equal to 300 KB, compressed
• Total execution time of less than or equal to two seconds
• Main-thread blocking of less than or equal to 200ms

Regularly review these metrics in Lighthouse’s Performance Budget section and track historical trends using Google Search Console’s Core Web Vitals reports. 

Once you’ve tackled the JS problem, shift your attention to event handlers. High-frequency events like scrolling or typing can easily overwhelm a browser if unmanaged, so evaluate where multiple handlers perform redundant tasks (such as animations, logging, or DOM queries) and consolidate them.

At the same time, use passive event listening where applicable and ensure listeners are removed when necessary. Many slowdowns on Shopify and BigCommerce stem from persistent listeners that are tied to interactive product widgets because they remain active after a user has begun navigating the page. 

Enhance Your Image and Asset Pipeline

Images remain one of the most influential factors in both LCP and INP scores. Poorly optimized media can block interactivity by consuming bandwidth and competing for main-thread parsing. 

As such, you’ll want to adopt a modern image pipeline that does the following:

• Serves WebP or AVIF from native CDNs
• Uses responsive image sizes and dimension attributes to stabilize the layout
• Prioritizes above-the-fold assets with explicit hints and lazy-loads the rest

Here’s a priority hints and lazy loading example:

<link rel=”preload” as=”image” href=”{{ featured_image | img_url: ‘master’ }}” imagesrcset=”{{ … }}” fetchpriority=”high”>

<img src=”{{ featured_image | img_url: ‘600x’ }}” loading=”lazy” decoding=”async” width=”600″ height=”600″ alt=”{{ product.title | escape }}”>

The above code will help you prioritize the right images while putting below-the-fold content on hold, thus enabling you to reduce LCP and improve INP simultaneously.

Making Theme and Template Fixes

Next, you’ll need to fine-tune the theme or template of your ecommerce store. When working on Shopify, that means you should:

• Inline the most important CSS for above-the-fold rendering
• Defer or remove legacy scripts included by old theme versions
• Audit Liquid sections to remove unused blocks that inflate DOM size

In BigCommerce, on the other hand, you’ll want to do the following:

• Inline essential style rules affecting the hero banner or top navigation
• Defer widgets such as carousels or review sections until after the first interaction
• Streamline theme.js by modularizing features and loading theme-only where necessary 

Admittedly, implementing some of these changes can be tough without the right technical skills. If you aren’t comfortable implementing these improvements on your own, partner with an experienced technical SEO firm that understands the challenges facing ecommerce brands.  

Accelerate Ecommerce CWV Improvements With Net Profit Marketing 

Working to improve INP in Shopify and BigCommerce requires a multifaceted approach. Trimming code is a good place to start, but you also need to build a JS budget and use event handlers to protect responsiveness. 

If you need expert guidance on implementing these optimizations and achieving lasting ecommerce CWV improvements that drive visibility and conversions, turn to Net Profit Marketing. Our team helps ecommerce brands benchmark, audit, and implement measurable CWV improvements at scale. Contact us to schedule a consultation with our team or book an audit today.