How fast is my store, and what's slowing it?
Bilby's Speed check measures Core Web Vitals on every page it visits — LCP, CLS, a TBT proxy from long tasks, TTFB — on desktop (1440×900) and mobile (390×844), records the transfer size of every request, and attributes the ten heaviest requests to the app or vendor that served them. Measurements are taken before any of Bilby's own instrumentation touches the page, from the store's home region, and compared against every store Bilby has visited (percentile printed with n).
What the Speed check catches
- Mobile LCP over 4 s on the home, collection or product page (P2)
- Desktop LCP over 4 s, or a page over 5 MB excluding streaming video (P3)
- The mobile homepage jumping around while it loads (CLS ≥ 0.25, P3)
- The main thread blocked by scripts, render-blocking scripts in <head>, and page weight by vendor
- What got slower this week and which script changed (with the Change check)
Severity — the rule, not a feeling
Questions people ask
Why do your numbers differ from PageSpeed Insights?
PSI is a lab run from Google's location with a throttled connection; Bilby measures a real Chromium session from your market's region with no throttling, on the exact pages a shopper visits — including the product and cart pages PSI never sees. Use PSI for the score; use Bilby for what shoppers experience tonight and what changed since last night.
Does video count in page weight?
No. A hero video keeps downloading for as long as we watch, so it is reported separately ("plus 8.6 MB of video") rather than counted in the page weight that drives a finding.
Read next: Slow after uninstalling apps? Leftover code · Leftover app code scanner · Shopify store not working on mobile
// SEE IT ON YOUR STORE
The free scan runs the Speed check tonight — the whole report, nothing locked.
Scan my store free