BigCommerce performance optimisation
BigCommerce handles hosting reliably, but theme JavaScript, app scripts, and unoptimised product images still slow your store down where it matters most — on mobile, on category pages, and at checkout.
Slow category and product pages on BigCommerce mean shoppers leave before they add to cart — and Google ranks faster stores above you in the results they are clicking.
For BigCommerce Essentials and Enterprise stores that need measurable speed improvements.
BigCommerce's managed infrastructure removes server configuration headaches, and its global CDN handles static asset delivery reliably. But the platform's performance ceiling is determined by what you build on top of it: your Stencil theme, installed apps, and the images in your product catalog. These factors are entirely within your control — and they are where most BigCommerce stores accumulate performance debt.
The BigCommerce App Marketplace makes it straightforward to add features: product reviews, loyalty programmes, upsell tools, live chat, and shipping calculators. Each app typically injects its own JavaScript on every page through BigCommerce's Script Manager. A store with eight apps loading individually means eight separate network requests, eight scripts competing for the main thread, and eight potential sources of layout shift or interaction delay. The cumulative cost often does not appear until a PageSpeed audit exposes it.
Stencil theme JavaScript bundles deserve specific attention. The default theme ships with a reasonable baseline, but themes that have been customised significantly — or custom themes built by agencies — often accumulate JavaScript from multiple sources without a coordinated approach to loading. Polyfills written for old browsers run on Chrome. Utilities included at project start but never removed are bundled into production. Widget Builder sections add CSS globally even when the widget is only used on one page template.
Product images are the most common LCP problem on BigCommerce stores. Images uploaded at full camera resolution — often five to fifteen megabytes — are served through BigCommerce's CDN with automatic resizing, but only down to the image dimensions your theme requests. If your theme requests a 1,200px wide thumbnail but your shopper is on a 375px wide phone, they receive an image three times larger than their display can show. Without responsive srcset markup in the theme, every mobile visitor pays the full bandwidth cost.
8+
scripts on the average BigCommerce store
Most mid-market BigCommerce stores have eight or more third-party scripts loading on product and category pages — analytics, marketing, chat, and reviews — each competing for the main thread.
3×
image weight without responsive markup
Without srcset attributes in the theme, mobile visitors download images sized for desktop — typically three times more data than needed for their viewport width.
60–70%
of ecommerce traffic is mobile
The majority of BigCommerce store visits arrive on mobile devices, where every extra kilobyte and millisecond of script execution has a disproportionate impact on the shopping experience.
Front-end delivery, image pipelines, and app script management — without breaking your theme.
We audit every script registered in BigCommerce's Script Manager, identify which page types each script is needed on, and configure non-critical scripts to load asynchronously or with a page-load delay. Marketing pixels are consolidated through Google Tag Manager where possible, reducing the total number of independent script requests on each page.
We update Stencil templates to use BigCommerce's CDN image resizing parameters and add srcset attributes so mobile devices receive appropriately sized images. Category grid thumbnails, product page images, and carousel assets are all given viewport-appropriate size targets, reducing image transfer size by 50–70% for mobile visitors.
We profile the Stencil JavaScript bundle to identify polyfills, unused utilities, and duplicate dependencies. Browser polyfills written for Internet Explorer are removed for modern browsers. Third-party scripts bundled into the theme are extracted and loaded through Script Manager for better control. Bundle size is reduced without removing functionality.
Product images are processed for correct compression and format before upload. We provide a batch processing workflow for the existing catalog and guidelines for future uploads. Combined with the CDN image API configuration, mobile shoppers see product images load in under a second on most store pages.
Widget Builder regions that load CSS globally are audited for templates where the widget is not displayed. CSS is scoped to the templates that actually use each widget, removing the overhead on product, category, and home page templates that do not render the widget.
Loyalty programme scripts, review widgets, and upsell tools are configured to load after the page is interactive where the app's configuration allows it. For apps that do not support deferred loading natively, we implement facade loading so the widget initialises on user interaction rather than blocking page load.
We reduce the number of font families and weights loaded per page, add preload hints for brand fonts used in the hero section, and implement font-display settings to prevent invisible text during load. Global CSS from the theme is audited for unused rules and reduced where the theme's selector structure allows safe removal.
We improve LCP on category and product hero images through responsive markup and Script Manager configuration; INP on add-to-cart, quantity, and product filter interactions by reducing main-thread script blocking; and CLS from price badge injections, promotional banners, and lazy-loaded images without reserved dimensions.
BigCommerce provides several built-in tools that store owners underuse. Script Manager is the most important: it gives you control over when and where each third-party script loads, including the ability to set scripts to load on specific page types only, defer or async loading, and control execution order. Most stores have analytics, live chat, review widgets, and marketing pixels all loading at document start on every page. Moving non-critical scripts to deferred loading via Script Manager is a zero-code change that can reduce LCP by 300–500ms on many stores.
BigCommerce's image API also offers on-the-fly resizing through URL parameters. When your Stencil theme requests product images, it can append width parameters to the CDN URL to get correctly sized images for each context. A well-configured theme requests 300px thumbnails for category grids and 800px images for product detail pages, and uses srcset attributes to serve appropriately sized images at each viewport. Many customised themes hardcode a single image size, missing this built-in capability entirely. Adding responsive image markup to Stencil templates is typically a theme edit that requires no platform changes.
For stores on BigCommerce Enterprise, headless options including Next.js Commerce and custom React front-ends are available. These architectures can produce substantially better PageSpeed scores than Stencil-based themes because they allow full control over JavaScript bundling, server-side rendering, and CDN caching strategy. The trade-off is implementation complexity and ongoing maintenance. For stores considering headless, performance is a strong argument in its favour — but only if the implementation is done with performance as a first-class concern from the start.
Representative outcomes from optimization projects. Your results depend on stack and traffic.
Home furnishings brand
Before
Mobile score 36
After
Mobile score 74
Responsive image implementation and theme JS audit
Sports equipment store
Before
LCP 4.6s
After
LCP 2.0s
Hero image and font preloading via Script Manager
Multi-channel retailer
Before
Checkout INP 420ms
After
Checkout INP 190ms
Script deferral and marketing tag consolidation
We run a deep speed and Core Web Vitals audit on your key templates — home, collection, product, and checkout where relevant. Every issue is ranked by its impact on real user experience and revenue.
We implement prioritised fixes: images, scripts, caching, and theme or app bloat — focused on revenue pages first. Each change is tested in staging and measured against a baseline.
You get ongoing checks so regressions from new apps, themes, or campaigns are caught before they hurt SEO and sales. Alerts fire when scores drop below your target threshold.
Explore our speed optimization services
Straight answers about timelines, risk, and what to expect.
PageSpeed optimisation service to increase your PageSpeed Insights score on mobile and desktop. Fix low scores with a clear, prioritised plan backed by Lighthouse data.
Learn more →Core Web Vitals fix service to improve LCP, INP, and CLS scores. We diagnose failing metrics and implement changes that help SEO and real-user experience.
Learn more →Magento 2 speed optimisation to reduce load times, pass Core Web Vitals, and improve ecommerce conversion rates. Audit-first, with measurable before and after results.
Learn more →Run a free speed test on your store URL. See exactly what is slowing you down.