How to Clean Up JSON/CSS to improve Store Speed?

Technical Q&A
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Topic summary

A store owner using a modified Sense theme achieved their desired appearance and functionality but faces a critically low site speed score of 32, despite optimizing images and removing unused apps. Standard CSS cleanup tools don’t work with Shopify, prompting a request for solutions that won’t compromise the store’s design.

Another user with the Impulse theme seeks help with similar performance issues, specifically:

  • Removing unnecessary JavaScript and CSS
  • Fixing an unload event flagged by Lighthouse
  • Resolving two errors in advanced-content.liquid:
    1. Replace lazysizes with native loading="lazy" attribute
    2. Add missing width and height attributes to image tags

The user shared their current image rendering code, which uses lazysizes library for lazy loading. A developer offered to review the first user’s store URL to provide specific recommendations. The discussion remains open with no solutions provided yet.

Summarized with AI on October 31. AI used: claude-sonnet-4-5-20250929.
1

I finally got my store looking and behaving the way I want. HOWEVER, my site speed is a 32. This is after optimizing images, etc. I’ve removed all unused apps, etc.

All of the CSS tools I’ve found (CSS Purge, etc.) don’t work with Shopify.

What is the best way to go about this without sacrificing appearance and functionality?

Store is a modified version of the Sense theme.

2

Hello,

Can you please share your store URL with us ?

3

Hi Rajvee,

Could you help me improve my store speed by removing unnecessary java script and CSS codes?

Also, in lighthouse, it’s showing my main page is using an unload event but I’m not sure how to fix this in code.

Final thing, in my advanced-content.liquid it is showing these 2 error messages 1) use the native loading=“lazy” attribute instead of lazysizes 2) missing width & height attributes on image tag

This is the code currently in my advanced-content.liquid. How do I fix this to remove the 2 errors? I appreciate any help. (I’m using the Impulse theme)

{%- if block.settings.image != blank -%}

{%- assign img_url = block.settings.image | image_url: '1x1' | replace: '_1x1.', '_{width}x.' -%} {{ block.settings.image.alt }} {{ block.settings.image.alt }}
{%- else -%}