There are several ways to test your Shopify store's page speed and they give different results, which causes confusion. This guide explains which tool to use, how to read the output, and what the numbers actually mean for your business.
The tools worth using
Google PageSpeed Insights (pagespeed.web.dev) — the most important tool. Enter any URL from your store and it returns a Lighthouse performance score (0-100) alongside specific metrics and improvement recommendations. Run this on your homepage, your best-selling product page, and a key collection page.
The critical setting: always check the Mobile tab, not Desktop. Google uses mobile performance for ranking signals. Desktop scores are typically 20-30 points higher than mobile. The mobile score is the number that matters for SEO and is the more realistic representation of what your majority-mobile visitors experience.
WebPageTest (webpagetest.org) — more detailed than PageSpeed Insights. Shows a waterfall diagram of every resource that loads on your page, how long each takes, and what's blocking others. Better for diagnosing specific performance problems once you've identified there's an issue. Use this when PageSpeed Insights identifies a problem but you need more detail to understand the cause.
Shopify's built-in speed report — in Shopify admin under Online Store - Themes - your theme - View report. Shows a speed score and comparison to other Shopify stores. This score is different from Google's Lighthouse score and is a Shopify-specific metric. It's useful context but Google's score is the one to optimise for.
Understanding the Lighthouse score
The Lighthouse score (0-100) is a composite of six metrics weighted differently:
- Largest Contentful Paint (LCP) — 25% weight: How long until the largest visible element on the page loads. Usually your hero image or main product image. Google's threshold: under 2.5 seconds is "Good," 2.5-4 seconds is "Needs Improvement," over 4 seconds is "Poor."
- Total Blocking Time (TBT) — 30% weight: How long JavaScript blocks the browser from responding to user input. High TBT = the page looks loaded but clicks and taps don't work. Usually caused by large third-party scripts.
- Cumulative Layout Shift (CLS) — 15% weight: How much the page layout shifts while loading (images without dimensions, fonts loading late, ads pushing content around). Score should be under 0.1.
- First Contentful Paint (FCP) — 10% weight: When the first content appears on screen. Under 1.8 seconds is Good.
- Speed Index — 10% weight: How quickly the page's content is visually populated overall.
- Time to Interactive (TTI) — 10% weight: When the page is fully interactive (all JavaScript has loaded and processed).
A score of 90+ is excellent. 70-89 is good. 50-69 needs attention. Below 50 is actively affecting conversion rate and rankings.
Reading the specific recommendations
Below the score, PageSpeed Insights lists specific opportunities to improve performance. The ones most commonly relevant for Shopify stores:
"Serve images in next-gen formats" — your images aren't in WebP format. Shopify serves WebP automatically when you upload to the admin, but if you're using external image hosting or CDN links, convert to WebP first.
"Properly size images" — images are larger than needed for their display size. A product image displayed at 400px wide but uploaded at 2048px wide downloads 5x more data than necessary. Shopify handles resizing natively; if this still appears, check for hardcoded image sizes in theme code.
"Eliminate render-blocking resources" — JavaScript or CSS files loading synchronously are blocking the page from rendering. Usually third-party app scripts. Fixing this requires moving scripts to load asynchronously - a developer task.
"Reduce unused JavaScript" — JavaScript is loaded but never executed on this page. Common with themes that include code for sections not used on this particular page. A developer can tree-shake unused code.
What the score means for your business
The relationship between Lighthouse score and business outcomes:
- Score below 50: Every additional second of load time reduces conversion rate by approximately 1%. A store loading in 8 seconds converts significantly worse than one loading in 3 seconds to the same visitor.
- Score 50-70: Performance is a factor in conversion rate and rankings. Improvement is worth the investment.
- Score 70-90: Performance is good and not a primary conversion barrier. Other CRO opportunities (product page, checkout, trust signals) will have higher ROI than further speed work.
- Score 90+: Performance is not a meaningful limitation. Don't spend development time chasing marginal improvements here.
How often to test
Test after any significant change to your store: installing or removing an app, changing themes, adding new page sections, or updating images. Apps are the most common source of performance regression - a new app can drop your score by 10-15 points. Test before and after any app installation so you know immediately if it's causing a problem.
Monthly testing as a routine check is reasonable for stores above £10k/month revenue where performance has material commercial impact.
