Anthony McGrathHow the speed checker works
The checker asks Google PageSpeed Insights to load your page on a mid-range phone (or a desktop) and measure it. Where Google has enough real Chrome visitors to your site, it also shows how fast the page loads for them over the last 28 days. That real-visitor figure is used first, because it is what your customers actually see.
The load time used is Largest Contentful Paint (LCP): the moment the main image or headline appears. Every second over your target is counted as lost sales.
Extra seconds = load time − target (2.5s)
Revenue at target = monthly revenue ÷ (1 − sales lost per second)extra seconds
Revenue lost a month = revenue at target − monthly revenue
Worked example
A store makes £50,000 a month and loads in 4 seconds on mobile. That is 1.5 seconds over target. At 10% fewer sales per second: £50,000 ÷ 0.91.5 = £58,560. The slow page is costing about £8,560 a month, or about £102,700 a year. Getting to 3 seconds alone would win back about £5,500 a month.
Revenue lost by load time
| Load time | Lost a month | Lost a year |
|---|---|---|
| 3s | £2,705 | £32,456 |
| 3.5s | £5,556 | £66,667 |
| 4s | £8,561 | £102,728 |
| 5s | £15,067 | £180,809 |
| 6s | £22,297 | £267,566 |
| 8s | £39,256 | £471,069 |
What the research says
- Deloitte's Milliseconds Make Millions study found that a 0.1 second faster mobile site lifted retail conversions by 8.4% and spend by 9.2%.
- Portent's 2022 research found ecommerce conversion rates of 3.05% at 1 second, 1.68% at 2 seconds, 1.12% at 3 seconds and 0.67% at 4 seconds.
- Both point to bigger losses than this checker's default of 10% per second, so the figures here are deliberately cautious.
Key terms
- Largest Contentful Paint (LCP)
- When the biggest image or text block appears. Good is 2.5 seconds or less, poor is over 4 seconds.
- Interaction to Next Paint (INP)
- How quickly the page reacts to taps and clicks. Good is 200 milliseconds or less.
- Cumulative Layout Shift (CLS)
- How much the page jumps around as it loads. Good is 0.1 or less.
- Core Web Vitals
- Google's three main page experience measures: LCP, INP and CLS.
- Time to First Byte (TTFB)
- How long the server takes to start sending the page. Good is 0.8 seconds or less.
- Speed score
- Google's 0 to 100 score from a single lab test. 90 or more is good.
Common questions
How much revenue does a slow website lose?
It depends on your traffic, but the research is consistent: slower pages convert fewer visitors. Deloitte found a 0.1 second faster mobile site lifted retail conversions by 8.4%, and Portent found ecommerce conversion rates fall from 3.05% at 1 second to 0.67% at 4 seconds. This checker uses a more cautious 10% fewer sales for each second over 2.5 seconds, which you can change.
How is the revenue loss worked out?
We take your load time (Largest Contentful Paint), find how many seconds it is over the 2.5 second target, and apply your sales lost per second for each one. Revenue at target = monthly revenue ÷ (1 − loss per second) to the power of the extra seconds. Revenue lost is the difference.
What is a good load time for an online store?
Google's target is a Largest Contentful Paint of 2.5 seconds or less for at least 75% of visits. Over 4 seconds is rated poor. Most shoppers are on mobile, so the mobile result matters most.
What is Largest Contentful Paint (LCP)?
LCP is the time it takes for the biggest thing on the screen, usually the hero image or main product photo, to appear. It is the closest single measure to when a page looks loaded, and it is one of Google's three Core Web Vitals.
Why is my mobile result slower than desktop?
Google tests mobile on a mid-range phone with a slower 4G connection, which is closer to how many shoppers browse. Large images, apps and scripts hurt far more on a phone than on a laptop with fast broadband.
Why does my speed score change between tests?
Each test loads the page fresh, so server load, ads, apps and network conditions shift the result a little each time. Real-visitor data from Chrome, shown when Google has enough of it, is steadier because it covers the last 28 days.
Does site speed affect SEO?
Yes, a little. Core Web Vitals are part of Google's page experience signals. The bigger effect is on sales: faster pages keep more visitors, convert more of them and make your ad spend go further.
How do I speed up a Shopify or WooCommerce store?
Start with the fixes the checker lists first. The usual wins are smaller, modern format images (WebP or AVIF), removing apps and scripts you no longer use, loading third-party code later, a lighter theme, and good hosting or a CDN.