Why is my Shopify store so slow? 9 causes I find in speed audits

1. The theme loads all its code on every page #

Many themes, including some premium ones, put all of their features into one big JavaScript file (often called theme.js) and one big CSS file. That file contains the code for the product gallery, the cart drawer, the mega menu, sliders, pop-ups, filters and everything else.

So when a visitor opens a simple page, their phone may still have to download and process code for features that aren't on that page. I check the request waterfall and execution trace before splitting code: global CSS can delay rendering, while unnecessary JavaScript adds download, parsing and execution work and can delay interactions.

Signs this is happening on your store:

• Your theme has one very large theme.js or global JavaScript file loaded on every page.
• PageSpeed Insights shows "Reduce unused JavaScript" and "Reduce unused CSS" with large savings.
• Even simple pages, like your About or Contact page, score poorly on mobile.

Why it costs you sales: Every page is slower than it needs to be, especially on mid-range phones, which is where most of your buyers shop.

2. JavaScript is scattered across the theme #

Over time, more scripts get added: inline scripts inside sections, snippets from old apps, code pasted in by different developers. Each one may be small, but together they block the page from becoming usable and are hard to track.

Signs this is happening on your store:

• Your theme has many <script> tags spread across sections and snippets.
• You (or past developers) have pasted code into theme.liquid "just to make something work".
• Nobody is sure what some of the scripts do anymore.

Why it costs you sales: Scattered scripts slow down the page and make every future change riskier and slower.

3. Images aren't sized for each screen #

A phone doesn't need the same image file as a large desktop monitor. Properly set up images give the browser several sizes to choose from, so each screen downloads only what it needs.

Many stores just use whatever image setup the theme came with. Sometimes that means a phone downloads a 2000px-wide image to show it at 400px, or every image is loaded at the same size no matter where it appears.

Signs this is happening on your store:

• PageSpeed Insights shows "Properly size images" or "Serve images in next-gen formats".
• Your product and collection images look fine but take a while to appear on mobile.
• You upload very large images and rely on the theme to "handle it".

Why it costs you sales: Images are usually the heaviest part of a page. Oversized images make the whole store feel slow.

Read more: Product images that are high quality and fast →

4. The most important image is lazy loaded #

Lazy loading tells the browser to wait before loading an image until the visitor scrolls near it. That's great for images further down the page. But many themes and apps lazy load everything, including the main hero or product image at the top of the page.

That image may be the Largest Contentful Paint element, but text or another image can be the LCP element instead. If the trace confirms an above-the-fold image is the LCP element, lazy loading can delay its discovery or request. I check the loading attributes and request timing before changing them; below-the-fold images can still benefit from lazy loading.

Signs this is happening on your store:

• Your hero banner or main product image appears a moment after the rest of the page.
• PageSpeed Insights shows "Largest Contentful Paint image was lazily loaded".
• Your LCP is much worse than your First Contentful Paint.

Why it costs you sales: Buyers see an empty space where your main image should be, and Google scores your page as slow.

5. The same script loads more than once #

This one is surprisingly common. The same library or tracking script gets loaded twice or more: once by the theme, again by an app, and again from code pasted into the theme. Each copy adds weight, and it can also cause bugs and double-counted tracking events.

Signs this is happening on your store:

• PageSpeed Insights or your browser's network tab shows the same file loading more than once.
• Your analytics show duplicate events (like purchases or add-to-carts counted twice).
• Removing an app didn't remove all of its code.

Why it costs you sales: You pay the speed cost several times for the same thing, and your data becomes unreliable.

Read more: Duplicate tracking events: why purchases get counted twice →

6. Apps you no longer use still slow you down #

Some apps add storefront scripts and styles; others do not affect the storefront at all. Uninstalling an app does not automatically mean it left residual scripts. Depending on how it was integrated, its storefront code may be removed cleanly, while manually inserted snippets or older integrations may need cleanup. I confirm what is still requested or executed, check the app developer's uninstall instructions and preserve a theme backup before removing code.

Signs this is happening on your store:

• You've installed and uninstalled many apps over the years.
• Your theme has snippets named after apps you no longer have.
• PageSpeed Insights shows third-party scripts from services you don't recognize.

What to verify: A leftover snippet name alone is not proof of a performance cost. Confirm that unused code still loads or runs, and that no active feature depends on it. Shopify's uninstall guidance explains when extra cleanup may be needed.

7. Using apps for simple features #

Many stores install an app for things that could be built directly into the theme: a contact form, a size chart, a badge, a simple pop-up, a countdown. Some add third-party requests or JavaScript across the storefront. The impact depends on where they load and what they execute, not just how many apps are installed.

My rule is simple: if a feature can be built properly into the theme, I build it in the theme instead of adding another app. That can reduce dependencies, but custom code also needs maintenance. I compare the measured cost and required functionality before recommending a replacement.

Signs this is happening on your store:

• The trace shows app scripts loading on pages where their features are not used.
• Some apps only do one small thing, like a form or a badge.
• PageSpeed Insights shows "Reduce the impact of third-party code" with a long list.

What to investigate: Third-party requests can compete for bandwidth, and their JavaScript can occupy the main thread. Use the trace to identify the cost, then test whether an embed can be scoped, deferred or removed without breaking its purpose.

Read more: A lighter newsletter form or pop-up →

Read more: An app for every cart feature →

8. "Speed fixes" that only fool Lighthouse #

Some store owners want a higher PageSpeed score fast, and some developers deliver exactly that: a better score, but not a faster store. A common trick is to detect the Lighthouse test and hide scripts from it, so the test sees a light page while real visitors still get the slow one.

The score goes up. The store doesn't get any faster, and your money and time are wasted. A Lighthouse score is not a ranking score. Check real-user Core Web Vitals alongside comparable lab traces; a test-only improvement does not establish a better visitor experience or promise a ranking change.

Signs this happened on your store:

• Your PageSpeed score jumped after a "speed optimization", but the store doesn't feel faster.
• The Core Web Vitals report in Search Console didn't improve.
• The work was done very quickly, with little explanation of what was changed.

Why it costs you sales: You pay for a number instead of a faster store, and the real problems are still there.

I don't do this. When I optimize a store, I fix the real causes and check real-world results, not just the score.

9. When the theme itself is the problem #

Sometimes a theme can't be fixed without fighting it at every step. If it's built on years of added code, doesn't follow Shopify's current best practices, or loads everything everywhere, every new feature makes it slower. Optimizing it is like patching a leaking boat.

In those cases, I'll tell you honestly that optimizing your current theme isn't worth it, and that moving to a better foundation will save you money in the long run. I turn down speed optimization work when I can see it won't give you real results.

When I build a store, I usually start from Dawn, Shopify's own reference theme. It's a useful starting point for a simpler theme architecture, but the final result still depends on the sections, media, apps and custom code added to it. I check the resulting store rather than treating a theme choice as a speed guarantee. Then I add your design and features on top, the right way.

Signs your theme is the problem:

• It's an older (pre-Online Store 2.0) theme, or heavily modified over many years.
• Previous speed work only gave small or temporary improvements.
• Every new feature or app makes the store noticeably slower.