CLS Differs on Shopify and Page Speed Insights

Store Design
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Topic summary

Mismatch between performance metrics across tools on mobile: Shopify reports CLS at 0.41, with LCP “Good” (1.21s) and INP “Good” (72ms); PageSpeed Insights reports CLS 0.1 (good) but flags issues with TBT and LCP. Desktop metrics appear all green.

Key terms: CLS (Cumulative Layout Shift: visual stability), LCP (Largest Contentful Paint: loading performance), TBT (Total Blocking Time: main-thread blocking), INP (Interaction to Next Paint: responsiveness).

Discussion focus: How to measure accurately and what to prioritize for improvement when tools disagree. Whether following PageSpeed Insights recommendations will improve Shopify’s metrics.

Advice provided:

  • Use PageSpeed Insights but test in an incognito window to reduce effects from cookies and apps.
  • Recognize website speed varies over time; results can fluctuate.

Current status: No concrete optimization steps or resolution. Open question remains on which tool to prioritize and specific actions to reduce TBT and reconcile LCP differences.

Summarized with AI on December 16. AI used: gpt-5.
1

The CLS of my Shopify Store is currently at 0.41. Largest Contentful Paint is Good ( 1.21s ) and Interaction to Next Paint is Good too ( 72ms )

PageSpeed Insights show me that the CLS of my store is 0.1, which is good… But shows me that the issue are the TBT and LCP.

And btw, this is on mobile. On desktop all the metrics are green. Everything’s good apparently

How should I measure this? What should I do to really improve my stores’s performance and make sure everything is running as smooth as possible?

Should I just take the advice from PageSpeed Insights and on Shopify it will get better too consequently?

2

Hi @COZYREST

Your website speed changes regularly. I would use the PageSpeed but open your website in the incognito browser. Sometimes, cookies and apps affects the result

3

It runs smoothly still, for me