Insights in this post come from our CRO team's decade of experience working with eCommerce brands. Written by Sumedha Gurav and Abhishek Talreja. Reviewed by Harsh Vardhan.
Here's something we've noticed after years of running speed audits: the fastest BigCommerce stores rarely have the highest Lighthouse scores. What they actually have is fewer of the problems shoppers notice.
We've run hundreds of eCommerce audits at this point, and the same culprits keep showing up.
Slow product pages. Render-blocking scripts. Images that are way bigger than they need to be. Too many third-party apps. Core Web Vitals nobody's checked in months.
If your conversion rate has been sliding and speed isn't the obvious cause, it's worth ruling out the other usual suspects too.
We cover them in why your conversion rate might be dropping.
Dozens of those audits were of BigCommerce stores specifically, and every time we traced the slowdown back to a root cause rather than guessing at fixes.
That's how we ended up with the seven areas below. They're what actually move the needle on BigCommerce speed, and here's the rundown.
In this guide, we'll walk through each one: what's actually causing the slowdown, and exactly how you fix it.
Before we dive into the details, here's the fast version worth bookmarking.
Every line below maps back to one of the seven areas above. Treat it as your quick-reference checklist before we get into the specifics.
With the quick version out of the way, here is the detailed walkthrough of each layer, starting with your scripts. With that out of the way, let's get into the details, starting with your scripts.
Bloated JavaScript is usually the first thing we find in an audit. It's also often the single biggest fix available, which makes it a good place to start.
Every unused function your browser has to parse costs you real, measurable time before a shopper even sees your page.
Tree shaking is what developers call the process of removing dead code, the functions and libraries your theme loads but never actually uses.
Strip that out, and the bundle shrinks, which means your browser downloads it faster.
You can run the same exercise on your CSS. Google PageSpeed Insights will flag unused styles directly, and from there it's just a matter of combining your files and trimming the whitespace.
Preloading is how you tell the browser which elements deserve priority. On a typical homepage, that's your navigation bar and your hero image.
If you're not sure what a strong one even looks like, we've rounded up some solid hero image examples worth stealing ideas from.
Chain those requests so elements load in the order a shopper actually scrolls through the page. Otherwise you end up with hundreds of assets fighting over the same bandwidth at once.
Open your developer tools, refresh with Ctrl+Shift+R, and watch how the page actually paints itself. Whatever's blocking that process needs to load asynchronously instead.
Small scripts you can't defer should get inlined. Everything else gets pushed out of the critical path.
Here’s what the final report will contain:
“The report was deep and super insightful. Can’t believe it’s free.”
Logan Christopher CEO, Empire Herbs
Images typically make up well over half your total page weight. That's exactly why this layer tends to deliver the fastest, most visible win in any BigCommerce speed test.
JPEG is your default for product photography. It compresses well at high resolution, and it's the format most theme builders expect.
Switch to PNG only when you're dealing with hard edges or sharp contrast, since JPEG tends to introduce visible artifacts there.
Running Cornerstone 4.0 or newer? Turn on the srcset attribute and let BigCommerce automatically serve the right image size for each device.
And don't skip your alt tags. They help shoppers using screen readers, sure, but they quietly do double duty for your SEO too.
Lazy loading defers your heavier elements so the essentials load first. That keeps your critical rendering path short instead of long and clogged.
There's one rule you really can't break here: never lazy load anything above the fold.
We see this mistake constantly in our audits, and it's one of the most common reasons a Largest Contentful Paint score fails. The browser ends up waiting on an image it should've loaded immediately.
BigCommerce gives you a genuinely strong foundation to build on. Most of the speed problems we actually find sit in what merchants have bolted on top of it, not in the platform itself.
A content delivery network is what lets a shopper on the other side of the world still see your static content quickly, while your web server handles everything dynamic.
BigCommerce gives you a few CDN options to choose from, including Akamai and EdgeCast.
Pick whichever sits closest to your customer base, and it'll pay off just as much on Black Friday as it does on an ordinary Tuesday.
Set up your 301 redirects under server settings, so a visitor who types an outdated URL never lands on an error page.
You've got two options here. Manual redirects mean you update the destination yourself.
Dynamic redirects update automatically whenever the underlying product, category, or blog page changes, which saves you the maintenance headache down the line.
This is where most stores bleed speed without ever realizing it. Every third-party app adds JavaScript, and it adds up faster than you'd think.
Here's roughly what the common categories cost you:
Try installing an ad blocker on a test browser and see how much faster your own store loads without all the third-party clutter. In our tests, sites often load at least fifty percent faster this way.
From there, build a request map to catch long redirect chains hiding behind third, fourth, and fifth-party resources, and keep running Google PageSpeed Insights regularly so new bloat doesn't settle in permanently.
A cluttered site structure makes both your browser and your shoppers work harder than they should have to.
Too many top-level categories do two things at once: they confuse shoppers, and they slow your menus down.
Merge related categories into wider parent groups instead of listing every micro-category across your top bar.
Shoes and bags, for instance, can comfortably share an accessories parent without losing any clarity for the shopper.
Dynamic content renders through your main server instead of serving statically, and on a homepage, that difference shows up fast, usually as a jump in bounce rate.
The fix is a caching plugin that stores your frequently accessed content and images, so a returning visitor isn't waiting on the server to rebuild the exact same homepage every single time.
Once it loads fast, make sure it's actually built to convert. Our breakdown of a high-converting eCommerce homepage covers the layout side of that equation.
Every blog post is technically its own page on BigCommerce. List dozens of them as one long horizontal scroll, and you're sending far more requests to your server than most homepages actually need.
Paginate instead, and cache whichever posts readers keep coming back to.
A strong filtering system means shoppers only load the products they actually care about, not your entire catalog at once.
That frees up server bandwidth and keeps every request efficient, and it matters even more once your catalog grows past a few hundred SKUs.
Sometimes the fastest fixes don't live on the page at all. They live one level below it, in how your theme actually renders content.
Google AMP strips your HTML down to something genuinely lightweight.
Custom CSS is capped at 75,000 bytes per page, according to AMP's own documentation, and author-written JavaScript is blocked almost entirely, so only pre-approved AMP components get to run.
That restriction sounds harsh, but it forces short, efficient rendering paths, exactly what a mobile shopper on a patchy connection needs.
BigCommerce itself recommends this move, and for good reason.
Stencil supports Google AMP natively. It automatically optimizes your images, uses YAML front matter for faster rendering, and supports rich snippets that help search engines read your pages more efficiently.
Going headless means separating your front end from your back end entirely, so your pages load without waiting on backend processing they don't actually need.
It also frees you up to hand specific functions off to specialized tools, which usually shrinks your overall response times too.
A Progressive Web App works off browser caching. The first time a shopper visits, their metadata and site data get stored locally, so the next visit loads from that stored data almost immediately.
The gains here are real, not theoretical. Orange Polska rebuilt its mobile site as a PWA and saw loading times drop 30 percent, along with a 52 percent jump in mobile conversions, according to Google's own case study.
This layer rarely shows up on a generic speed checklist, but in our audits, it's consistently one of the highest-leverage areas we look at, simply because it's where the money actually changes hands.
Every script you add to checkout, whether it's a payment widget, a trust badge, or a live chat pop-up, competes for the exact same milliseconds a shopper needs to finish typing their card number.
Audit this page separately from your homepage. BigCommerce apps often load completely different assets depending on the template, so what you find on your homepage audit won't necessarily apply here.
Default to guest checkout, and cut any form field that isn't strictly necessary.
Fewer requests at this stage means fewer abandoned carts, plain and simple. For more on what that page should actually look like beyond speed, see our guide to cart page designs that convert.
Aim for a Largest Contentful Paint under two and a half seconds on mobile, which is the same Google Core Web Vitals threshold that applies to desktop. Your full page should load in under three seconds.
Mobile networks are just less forgiving than desktop connections. An image that loads fine on your office WiFi can stall out completely on four bars of cellular signal.
Test your product pages, category pages, and checkout flow separately, and do it on a throttled connection.
BigCommerce mobile speed problems rarely show up the same way twice across different templates.
If mobile checkout is still underperforming once speed is fixed, our mobile checkout optimization guide covers the friction points that have nothing to do with load time.
Here’s what the final report will contain:
“The report was deep and super insightful. Can’t believe it’s free.”
Logan Christopher CEO, Empire Herbs
Optimization isn't a project you finish once and forget about. Catalogs grow, apps pile up, and themes drift over time, and this layer exists to catch that drift before it costs you a quarter's worth of traffic.
Google measures your BigCommerce page speed through three specific Core Web Vitals.
Google's official thresholds spell out what counts as good: Largest Contentful Paint under 2.5 seconds, Interaction to Next Paint under 200 milliseconds, and Cumulative Layout Shift at 0.1 or below.
In plain terms, Largest Contentful Paint tracks how fast your main content shows up, Interaction to Next Paint tracks how quickly the page responds once someone actually clicks something, and Cumulative Layout Shift tracks how much your layout jumps around while it all loads.
These days, all three factor directly into your search rankings, not just your conversion rate.
Start with Google PageSpeed Insights. It's free and gives you a detailed breakdown of both your mobile and desktop scores.
From there, check Google Search Console under the Core Web Vitals report in the left sidebar.
That data comes from real Chrome users visiting your actual store, not a lab simulation, which makes it worth trusting more than a one-off test.
GTmetrix is worth adding to the rotation too, with a free plan and three paid tiers scaled to store size.
Pingdom offers a fourteen-day free trial with round-the-clock monitoring from seventy polling locations worldwide, and WebPageTest rounds things out with three hundred free monthly test runs across thirty locations, connection speeds, and browsers, plus a paid tier if you need priority or bulk testing.
Run each tool against your homepage, a category page, a product page, and your checkout flow separately.
BigCommerce templates rarely share identical bottlenecks, so testing just one page tells you less than you'd think.
In our experience, the merchants who stay fast are the ones who treat this like a recurring calendar item, not a one-time fire drill.
It's worth pairing this with a full UX audit checklist while you're in there, since the two tend to surface overlapping issues.
A handful of mistakes show up in nearly every audit we run. They tend to quietly undo progress you've already made elsewhere, which is what makes them so frustrating to catch.
Fix these, and if cart abandonment is still high afterward, the cause is probably not speed at all. See why your cart abandonment rate might still be high for the rest of the list.
The average online store converts less than two percent of its visitors into buyers, according to IRP Commerce's ecommerce benchmark data.
Do the math, and that means well over ninety-eight percent of visitors who land on an eCommerce site leave without buying anything. BigCommerce stores see that same pattern.
Speed just gets you in the door faster. What actually keeps a shopper there is the experience once they arrive, and that's exactly the problem ConvertCart solves.
We've helped over five hundred eCommerce stores across the US improve their user experience and double their conversions.
Our conversion experts can audit your site, flag the UX issues actually costing you sales, and hand you a plan to fix them.
If you're specifically on BigCommerce, our guide to improving BigCommerce conversion rate is a good next read once your speed fixes are live.
Aim for a Largest Contentful Paint under two and a half seconds, with your full page loading in under three seconds on mobile and closer to two on desktop.
Go slower than that, and you'll start losing conversions in a way you can actually measure.
Not for most of it. Image compression, app audits, redirect cleanup, and CDN configuration are all things you can do without writing a line of code.
Theme-level changes, like adopting Stencil or going headless, usually do benefit from developer support, so it's worth knowing which category you're dealing with before you dive in.
Quarterly at minimum, plus right after you install any new app or switch themes.
Speed tends to degrade gradually as a catalog grows, so if you wait until a shopper complains, you've already lost sales you'll never see reported anywhere.
Image optimization, hands down, in nearly every audit we run. Compressing and correctly formatting your images typically delivers the biggest visible improvement for the least amount of engineering effort.
For the bigger picture on what moves conversion rate beyond speed, our conversion rate optimization statistics roundup is a good next read.
It's still useful, particularly for content-heavy mobile pages like blog posts and top-of-funnel landing pages.
It matters less for product and category pages, though, since BigCommerce's native Stencil optimizations already do much of the same work there.
Speed isn't a box you check once and move on from. Revisit these seven layers every time your store changes, because it's always changing in some way.
Start wherever your last PageSpeed Insights report told you to start, and work through the rest from there.
You'll feel the difference before your dashboard even confirms it. Your shoppers already have.
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