I have a Shopify site and I’m trying to debug some speed issues. When I test my site using tools like PageSpeed or GTmetrix, they store a cached version, and I can’t test again for about an hour. I know this happens because I tried commenting out the entire page and testing again, but the test tools still show the previous version.
Since we can’t change cache headers on Shopify (unless there’s a workaround I don’t know about), please let me know if it’s possible.
Also, I need ways to debug the site instantly without waiting for these tools to refresh their cache. Even after optimizing things, the JavaScript payload still appears very large. Is this a Shopify limitation, or is there something I can fix on my end? I’m struggling to get the mobile score above 65, and it still fluctuates a lot.
Analytics
Generally, each theme code edit should break server-side cache, so it’s not a Shopify fault IMO.
There is a Lighthouse tab in Chrome Developer tools which does very similar thing.
Another nice tool in Chrome dev tools is on a “performance” tab.
I already tried chrome developer tool for debugging. I also experienced cache issue there as cache is stored for several days and the site do not represent current change. I need to test in incognito tab each time and still chrome lighthouse don’t represent actual metrics. It always shows my site is 90+ even though it is not. [pagespeed](http://pagespeed.web.dev/) represent somewhat real experience but it has cache issue.
Hi @user2937 ,
You’re correct that PageSpeed Insights, GTmetrix, and similar tools often cache recent test results, especially when you run tests multiple times within a short period. This is normal behavior, and it’s not related to your Shopify store specifically.
a. Use Chrome DevTools → Lighthouse (local run)
This runs directly on your device — no caching, no CDN delay.
Steps:
- Open your site in Chrome
- Press CTRL + SHIFT + I
- Go to Lighthouse → Mobile → Generate Report
This gives the most immediate and accurate feedback.
b. Chrome DevTools → Performance Tab
You can record the full load and see:
- JavaScript execution time
- Layout shifts
- Network waterfalls
- Blocking scripts
c. Use “Disable cache” in Chrome DevTools (Network tab)
This forces your browser to fetch fresh assets every reload.
You do not really need absolute values every 5 minutes.
You change something and you need to know whether this helped – so you can compare your local runs before and after the change to confirm this.
Also – real experience, real CVW metrics come from the visitors browsers, not from the Pagespeed server.
From the Chrome users like yourselves.
This is a very relatable issue for many Shopify store owners. The caching behavior of speed testing tools makes it hard to see real-time changes, especially during active debugging. It’s also frustrating when scores fluctuate even after making improvements. The concern about large JavaScript payloads is valid, and many merchants face the same challenge. Discussions like this help highlight where Shopify could offer better visibility and control for performance testing.
Hi @user2937
PageSpeed and GTmetrix cache your site for about one hour, and you cannot disable that on Shopify because Shopify controls all cache headers. This is normal and not something you can change.
To test your changes instantly:
- Use Chrome DevTools Lighthouse. This always loads a fresh version.
- Test your theme preview URL. Preview links are never cached.
- Use WebPageTest, which does not cache your site.
Large JS payload is common on Shopify because of built in scripts and app scripts. You can reduce it by removing unused apps, turning off unused app embeds, and deferring non essential JS. Getting stable mobile scores above 70 on Shopify is possible but scores will still vary.
Best Wishes
