A slow-loading ecommerce store can frustrate shoppers, reduce engagement, and make product discovery more difficult. On BigCommerce, Stencil themes provide the foundation for storefront design and functionality, but poorly optimized images, scripts, and layout components can affect performance. Core Web Vitals help identify these issues by measuring loading speed, visual stability, and responsiveness. Understanding common theme-related mistakes allows store owners to improve the shopping experience without unnecessarily rebuilding their storefront.
Understanding Core Web Vitals in BigCommerce Stencil Themes
Core Web Vitals are Google’s metrics for evaluating important aspects of the user experience. For BigCommerce storefronts, these metrics help developers identify problems that affect page loading, visual stability, and interaction responsiveness.
- Largest Contentful Paint (LCP): Measures how quickly the largest visible content element loads. A good LCP score is 2.5 seconds or less.
- Cumulative Layout Shift (CLS): Measures unexpected movement of page elements during loading. A good CLS score is 0.1 or less.
- Interaction to Next Paint (INP): Measures how quickly a page responds visually to user interactions. A good INP score is 200 milliseconds or less.
These metrics should be evaluated using real-user data at the 75th percentile, with mobile and desktop performance considered separately. Effective BigCommerce development helps identify theme-related performance issues and implement targeted improvements to loading speed, visual stability, and overall storefront responsiveness.
7 Stencil Theme Mistakes Hurting LCP and CLS
BigCommerce Stencil themes support storefront customization, but unnecessary modifications and inefficient implementations can introduce performance problems. The following mistakes are common areas to investigate when a store fails its Core Web Vitals assessments.
1. Using Oversized Hero Images
Large hero banners and product images can delay LCP because they often occupy a significant portion of the initial viewport. When these assets are larger than necessary, mobile shoppers may experience longer loading times, particularly on slower connections.
How to fix it:
- Compress images without noticeably reducing visual quality.
- Use modern formats such as WebP or AVIF where supported.
- Serve responsive image sizes based on device requirements.
- Prioritize the main above-the-fold image and avoid lazy-loading the LCP image.
Optimizing image delivery can reduce unnecessary data transfers and help the main content render sooner.
2. Loading Too Many JavaScript Files
Third-party applications, analytics tools, chat widgets, promotional pop-ups, and custom scripts can add unnecessary JavaScript to a storefront. Excessive script execution may delay rendering and compete for browser resources, affecting LCP and potentially worsening INP.
How to fix it:
Audit installed applications and remove scripts that no longer serve a clear business purpose. Defer non-critical JavaScript when appropriate, and load optional functionality only when necessary. Review custom Stencil theme scripts to identify duplicate dependencies and unnecessary execution.
The objective is not to eliminate useful integrations but to ensure that non-essential functionality does not interfere with the initial shopping experience.
3. Adding Custom CSS That Blocks Rendering
Custom styling can improve storefront design, but excessive CSS overrides, unused rules, and multiple stylesheets may increase the work required before the browser can render a page. This can delay the display of important content and contribute to slower LCP.
How to fix it:
Review custom stylesheets for redundant selectors, unused CSS, and conflicting declarations. Remove unnecessary rules and optimize how critical styles are delivered. Test changes across mobile and desktop breakpoints to ensure that performance improvements do not break product layouts, navigation, or responsive components.
Effective BigCommerce theme optimization requires balancing visual customization with efficient rendering.
4. Failing to Reserve Space for Images and Banners
When images, promotional banners, or embedded components appear without predefined dimensions, surrounding content may move after the page begins rendering. These unexpected movements increase CLS and can cause shoppers to lose their place while browsing products.
How to fix it:
Define image width and height attributes or use CSS aspect ratios to reserve space before assets load. Ensure promotional banners, product cards, and dynamically inserted elements have predictable dimensions. For components that load asynchronously, allocate sufficient space in advance.
Stable layouts make the storefront easier to navigate and reduce disruptive visual movement during loading.
5. Loading Web Fonts Inefficiently
Using several font families, numerous font weights, and large font files can delay text rendering. Font swapping may also change text dimensions, moving buttons, navigation elements, and product details after the page initially appears.
How to fix it:
Use only the font families and weights required by the storefront. Prefer optimized WOFF2 files where supported, and configure appropriate font-display behavior. Choose fallback fonts with similar character dimensions to reduce layout differences during font swapping.
Font optimization is particularly important for storefronts where headings, prices, navigation, and promotional messages occupy prominent positions.
6. Using Heavy Sliders and Animation Effects
Autoplay sliders, large promotional carousels, and complex animations can increase rendering work and introduce additional scripts or image requests. If the main hero content depends on a carousel script, its rendering may also be delayed.
Animations that change layout properties, such as height or margins, can cause visual instability when elements move unexpectedly.
How to fix it:
Replace unnecessary carousels with a single optimized banner when multiple slides do not provide meaningful value. Reduce animation complexity and prefer transform or opacity for suitable visual effects. Ensure that carousel containers reserve adequate space before their content loads.
These changes can simplify rendering while keeping the storefront visually engaging.
7. Ignoring Mobile-Specific Theme Optimization
A storefront that performs well on desktop may still struggle on mobile devices. Oversized images, complex navigation, excessive above-the-fold content, and poorly configured responsive elements can slow loading and create layout shifts on smaller screens.
How to fix it:
- Test mobile and desktop layouts independently.
- Deliver appropriately sized images for smaller screens.
- Simplify above-the-fold content where possible.
- Check that menus, product grids, banners, and buttons maintain stable dimensions.
- Remove unnecessary mobile scripts and visual effects.
Mobile performance deserves particular attention because real-user Core Web Vitals data can differ substantially between mobile and desktop visits.
How to Diagnose Core Web Vitals Issues in BigCommerce
Identifying the source of a performance problem is more effective than making random theme changes. Several tools can help developers investigate the underlying causes.
- Google PageSpeed Insights: Provides LCP, CLS, and INP assessments, along with diagnostic recommendations and available field data.
- Google Search Console: Groups URLs with similar Core Web Vitals issues when sufficient real-user data is available.
- Chrome DevTools: Helps identify slow network requests, rendering delays, JavaScript execution, and layout shifts.
- BigCommerce Stencil tools: Support theme previewing and testing before changes are deployed to the live storefront.
Test the homepage, representative product pages, and category pages on both mobile and desktop. Use lab diagnostics to investigate specific problems, then monitor field data to assess real-world improvements.
Best Practices to Improve LCP and CLS in BigCommerce
Consistent performance optimization helps prevent theme customizations from introducing new problems. Store owners should prioritize the largest content element, compress images, remove unnecessary scripts, minimize redundant CSS, and reserve space for dynamically loaded components.
As part of ongoing BigCommerce store optimization, retest Core Web Vitals after significant theme updates, new application installations, and changes to promotional content. Evaluate both lab results and real-user field data before concluding that a change has improved performance.
Conclusion
Poor Core Web Vitals scores do not automatically indicate a platform-level problem. Oversized assets, excessive scripts, inefficient styling, and unstable layouts are common issues to investigate. With targeted fixes and regular testing, store owners can improve loading performance and visual stability. With 12+ years of experience, EmizenTech provides BigCommerce development services to help businesses optimize Stencil themes, resolve performance bottlenecks, and deliver a smooth shopping experience.
Frequently Asked Questions
1. Why does my BigCommerce store have a poor LCP score?
Common causes include oversized hero images, slow server responses, render-blocking resources, and delayed rendering of the main visible content.
2. Can a BigCommerce Stencil theme affect CLS?
Yes. Missing image dimensions, dynamically inserted banners, font swapping, and unstable product layouts can cause unexpected content shifts.
3. How often should I test BigCommerce Core Web Vitals?
Test after major theme changes, application installations, and storefront updates. Regularly review field data to identify performance regressions and confirm whether fixes improve real-user experiences.
