Nobody decides to make a store slower. It happens a little at a time, in changes that each looked harmless. The usual cause is one of five things, and you can find which one in an afternoon.
Stores change speed every week
We re-measure every store we write about, usually a week after the first reading: phone-size screen, empty cache, the way a first-time shopper arrives. Ten established Shopify stores, measured twice:
Three got noticeably slower. Two got dramatically faster. The rest moved a little. One store went from 3.3 MB to 18.1 MB in six days, and its main image went from 5.1 to 8.5 seconds. Nobody redesigned it. Something was added.
The two that got faster prove the other half: the same stores, fixed within weeks, without a rebuild.
The five usual causes
- A new app. Many apps add code to every page, even pages where they do nothing. Uninstalling an app often leaves its code behind in the theme.
- A new hero. Marketing swaps the homepage image for a campaign. The new one is a 5 MB file, or an autoplaying video, and it is the first thing every visitor waits for.
- A popup or countdown. Email capture, spin-to-win, free shipping bars. Each one is code that runs before the shopper can do anything.
- A new tracking tag. One more pixel, or a tag manager container that quietly grew to forty tags.
- A theme update. Usually faster, occasionally not, especially when old customisations were copied across.
How to find yours
- Find when it started. Search Console's Core Web Vitals report shows the trend over time. PageSpeed Insights shows the last 28 days of real visitors. A step change has a date.
- List what changed that week. In Shopify, check which apps were installed, and open your theme's code history: each file keeps its older versions. Then ask marketing what launched.
- Weigh the page. In Chrome, open the developer tools, go to Network, reload, and sort by size. The biggest file is often the whole answer. Then look at the domains: how much of the page comes from companies other than you.
- Test without it. Duplicate the theme, remove the suspect, and measure the copy. Do not test on the live theme.
Fixing it
Most of the fixes are small. Compress or resize the hero. Remove the leftover code from apps you no longer use. Hold chat, reviews and marketing scripts until the shopper first scrolls or taps, which we explain with code indelaying third-party scripts.
Then keep measuring. The stores in our chart did not get slow once. They drift, every week, in both directions. The ones that stay fast are the ones where someone looks.
If you want a second pair of eyes, send us your store address. We will measure it twice, a week apart, and tell you what changed.