Why does my mobile speed score keep fluctuating?

Technical Q&A

Topic summary

Mobile speed scores on a Shopify store fluctuate widely (reported range ~29–69) despite strong desktop results (GTMetrix A/97% performance; PageSpeed Insights 93–97). The main question is what causes the inconsistent mobile PageSpeed rating and how to keep it consistently high.

A response reviews the site using Lighthouse and GTmetrix and reports low performance on mobile. Suggested optimization targets include:

  • Upgrade/optimize the theme if possible.
  • Reduce JavaScript payload and execution (including third-party scripts and Shopify apps).
  • Implement critical CSS and address render-blocking resources.
  • Optimize images (reduce sizes, compress).
  • Preload key requests and web fonts; consider using a standard web font.
  • Improve TTFB (server response time), caching, avoid redirects, reduce DOM size, and minimize critical request depth.

Key metrics referenced: First Contentful Paint, Time to Interactive, Total Blocking Time, Speed Index, Largest Contentful Paint, and Cumulative Layout Shift (layout movement). No definitive root cause for the fluctuation is confirmed; the thread ends with general recommendations and an offer for a paid optimization plan.

Summarized with AI on February 26. AI used: gpt-5.2.
1

Hello,

Apologies in advance for the multiple posts over the past few days.
I’ve made significant progress in optimizing my store though, and just need a bit of a nudge in the right direction for the mobile speed.

I’ve managed to get my GTMetrix score to an A Grade (97% Performance / 93% Structure) for desktop.
My PageSpeed Insights score varies between 93-97 for desktop.

However, the mobile score can vary anywhere from 29 to a high of 69.
The last score before this post is 52 for mobile.
What is causing this fluctuation and how can I maintain a high rating consistently?

My store is https://www.killyourgod.ai

2

Hello @Blaine2045 ,

I have gone through your store. Your store looks good.

I have checked your site in the page speed tool. your website score is very low.

Lighthouse Score :

oscprofessional_0-1638352095763.png
oscprofessional_0-1638352095763.png1350×630 102 KB

GT Matrix Score :

oscprofessional_1-1638352095350.png
oscprofessional_1-1638352095350.png1192×671 88.3 KB

Need to work on following points

  • Theme Upgrade if there is a scope and upgraded theme available
  • Reduce JS Payload.
  • Use Critical CSS .
  • Reduce Large Image Sizes
  • Compress Images
  • Limit Third Party JavaScript & Shopify Apps
  • Preload Web Fonts.
  • I would suggest limiting to one and using a standard web font.

See where exactly google speed index tools want you to target your code to optimize the site .

First Contentful Paint

  • Eliminate render-blocking resources
  • Reduce server response times (TTFB)
  • Avoid multiple page redirects
  • Preload key requests
  • Avoid enormous network payloads
  • Serve static assets with an efficient cache policy
  • Avoid an excessive DOM size
  • Minimize critical request depth

Time to Interactive TTI :

Time to Interactive (TTI) is one of six metrics tracked in the Performance section of the Lighthouse report. Each metric captures some aspect of page load speed.

Total Blocking Time:- Reduce Unnecessary JavaScript loading, parsing, or execution.

  • Reducing JavaScript payloads with code splitting, removing unused code, or efficiently loading third-party JavaScript.

Speed Index- Minimize main thread work

  • Reduce JavaScript execution time
  • Ensure text remains visible during webfont load

Largest Contentful Paint

LCP is primarily affected by four factors:

  • Slow server response times
  • Resource load times

Cumulative Layout Shift

  • Cumulative Layout Shift measures the movement of visible elements within the viewport

By fixing these issues, your website speed score can definitely improve.
And we will provide an assured guarantee plan for the website speed optimization.

For more details please check the link below.

https://www.oscprofessionals.com/shopify-performance-optimization/

If anything is missed out or unclear then don’t hesitate to ask. Email us or Connect with us on Skype

Have a nice day !