I currently have a slow website and have been searching for answers over the last few months. I have read that old java scrip will often cause a website to run slow, so I would like to find out if this is the case with my website. Also, after optimizing photographs and alt text yesterday, the LCP P75 is running very slow at 4328 milliseconds. I have no idea how to fix this either. I need some help please. Marilyn
Speed checkers like https://pagespeed.web.dev/ give you an analysis on what is going on, and where to look to fix it.
Hi @Rainbird
If you’re looking to optimize your Shopify store’s speed, these can be great adjustments to make. Improving image optimization, reducing requests, and implementing lazy loading can be great ways to ensure your site loads faster.
1.Image Optimization.
2.Reducing HTTP Requests.
3.Implementing Lazy Loading.
4.Optimizing JavaScript and CSS.
5.Using Content Delivery Network (CDN).
6.Evaluate and Remove Unnecessary Apps.
7.Optimize Mobile Experience.
8.Regularly Monitor Performance.
Best regards,
Devcoder 
Hi, @Rainbird
Could you please share your website URL? I will help review it and analyze the reasons for the slow performance.
This may be a coincidence, but also – Shopify uses server side caching to serve your pages faster.
When you do changes like these, the cache is invalidated to reflect the new data.
Your pages are cached again afterwards, so it may worth checking back in a day or two whether the parameter is back to normal.
Hey @Rainbird
First, check which apps you have installed. Old or poorly coded apps are the biggest culprit for slow sites because they add JavaScript that runs on every page. Go through your apps and uninstall anything you’re not actively using. Even deactivated apps can leave code behind, so fully uninstall them.
For your LCP issue specifically, make sure your hero image or largest above-the-fold image is properly optimized. Even if you optimized photos yesterday, check the file size of that main image - it should be under 200KB ideally. Also make sure it’s in a modern format like WebP instead of JPEG or PNG.
Check your theme too. Some themes are just bloated with features you don’t need. If you’re on an older theme, consider switching to something lightweight like Dawn, which is Shopify’s free default theme and really fast.
Run your site through Google PageSpeed Insights and look at the specific recommendations. It’ll tell you exactly which scripts or images are causing the slowdown. Look for “reduce JavaScript execution time” or “eliminate render-blocking resources” in the diagnostics.
This is not necessary – Shopify automatically serves your images as WEBP if visitors browser supports this format even if JPEG is uploaded.
Thanks for sharing that! You’re absolutely right - I should have been clearer about that.
Even though Shopify converts to WebP, starting with a massive 5MB JPEG will still cause issues. Compressing images before upload to reasonable file sizes (under 300-500KB for hero images) is what really matters, regardless of the format.
Hi,
Hope this will help
Image optimization alone doesn’t fix LCP
- Yes, old and unused JavaScript often slows Shopify stores
- Deleted apps frequently leave scripts behind
- LCP is usually caused by hero image or above the fold content
Make changes step-by-step and retest each time
- Remove unused scripts
- Fixing hero image loading
- Simplifying the homepage top section
Hey Marilyn,
Could you share your store url so that I can take a look in the current issue with the loading speed and provide you the best suggestions on how to fix it.
Waiting to hear back from you.
A slow site is usually caused by a few common things, not just old JavaScript. In Shopify, unused apps, heavy theme scripts, and large hero images are often the biggest reasons for poor speed and high LCP. Even if images are optimized, the main banner or slideshow can still load slowly if it’s large or loaded late.
A good first step is to test your site in Google PageSpeed Insights and look specifically at what’s affecting LCP, usually the main image or heading above the fold. Try removing apps you no longer use, disabling unused features in your theme, and making sure your main images are compressed and not set as background images when possible. If the theme itself is heavy, switching to a lighter theme can make a big difference.
Good evening, my website it https://timelessfashions.co.uk/
Hi, @Rainbird
I’ve checked your website, and the LCP does indeed need optimization. To improve LCP, I suggest compressing images and enabling image preloading. For mobile, use a smaller image size.
Hi Rainbird,
You’re on the right track — JavaScript is very often the main cause of slow LCP on Shopify, especially when image optimization doesn’t move the needle.
A few important points:
-
“Old” JavaScript itself isn’t the issue — it’s usually too much JS, render-blocking JS, or apps injecting scripts site-wide.
-
LCP at ~4.3s (P75) after image optimization strongly suggests the LCP element is being delayed by JavaScript, not image size.
What to check first:
-
Identify the LCP element in Chrome DevTools (Performance or Lighthouse).
-
Look for apps loading JS on every page (reviews, popups, tracking, sliders).
-
Remove or disable unused apps — even deleted apps often leave scripts behind.
-
Make sure non-critical JS is deferred or loaded after interaction.
-
Check if your theme is JS-heavy (older themes are common culprits).
Quick wins:
-
Test by temporarily disabling apps (starting with marketing, reviews, popups).
-
Use PageSpeed Insights → “Diagnostics” to see which scripts block rendering.
-
Avoid sliders or hero videos above the fold.
It is possible to modify your theme to address high LCP.
At the moment, visitors browser need to load entire page HTML and only then it starts downloading images on the page.
Shopify has a mechanism to load images in parallel with the main page HTML, but it is not a used by Dawn.
With minimal changes you should be able to shave off about 0.5 seconds from your LCP.
However, this will require theme code edits => you’d need a developer.
Hi @Rainbird ,
Largest Contentful Paint (LCP)
- LCP is delayed because hero images, banners, or large product images load late.
- Preload LCP image
- Add fetch-priority=“high” on the lcp img tag
- reomve lazy load from lcp img tag.
Hi, @Rainbird
Start by checking your site with Google PageSpeed Insights to identify the elements most affecting performance. Slow website performance is usually the result of multiple small inefficiencies accumulating over time, rather than a single issue.
- Optimize images properly
Use the correct image formats (JPEG/WebP instead of PNG where possible) and let Shopify serve responsive image sizes. Avoid loading oversized images on mobile.
<img
src=“/images/product.jpg”
width=“800”
height=“600”
alt=“Product image”
loading=“lazy”
- Reduce HTTP requests
Limit the number of scripts, custom fonts, and third-party assets loading on each page. Fewer requests generally mean faster load times.
- Implement lazy loading
Lazy-load images, videos, and sections that appear below the fold so the browser can focus on rendering visible content first.
- Optimize JavaScript and CSS
Defer non-critical JavaScript and remove unused CSS. Render-blocking scripts are one of the biggest causes of slow mobile scores.
Load CSS efficiently:
-
Review and remove unnecessary apps
Apps often add hidden scripts. Uninstall anything not actively providing value and test performance after each removal. -
Optimize the mobile experience
Mobile devices are more sensitive to heavy scripts and large DOMs. Simplify layouts, reduce animations, and prioritize above-the-fold content.
Alternatively , If you’d prefer not to handle everything manually, you can also try Website Speedy, a Shopify speed optimization app that helps improve overall page speed and performance.
(Disclaimer: We are the developers of Website Speedy)