PageSpeed Insights
What is PageSpeed Insights?
PageSpeed Insights is Google’s free tool for measuring how fast a single page loads, with a score from 0 to 100 given separately for mobile and desktop. The score comes from a Lighthouse lab test; on mobile it simulates a mid-range phone on a slow connection. Above it, when the site has enough traffic, sit 28 days of data from real Chrome users, and that is what Google uses to assess Core Web Vitals.
Example
The manager of an accountancy firm runs the home page through PageSpeed Insights on her phone and gets 41. On her laptop it scores 88. An hour later the mobile test says 36. Nothing is broken. Mobile and desktop are two different tests, and the mobile one simulates a weaker phone on a slower connection, which is also where gaps in mobile optimisation show. A lab score also moves by a few points from run to run as the load on the server and the network varies. So we compare the same page on the same device, and run each test more than once.
The report has two halves. The top one shows 28 days of real-visitor data; on a site with little traffic it is missing. The bottom one is the lab test: the 0–100 score and a list of specific findings, such as heavy images, code that blocks the page from appearing, and third-party scripts. The score is a summary; the work is in the list.
Why it matters for a business
The score is the first thing a client or a competitor checks, but Google does not rank pages by it. Ranking and the Core Web Vitals assessment use the real-visitor data. The lab half is Lighthouse, a diagnosis that shows what slows the page and in what order to fix it. That is why a proposal that “guarantees 90+ in Lighthouse” is promising a number that also depends on the hosting and on the platform the site runs on. We name the device next to every number and compare mobile with mobile, desktop with desktop — a comparison across the two means nothing.
What to ask
- Which device is the number from? Mobile and desktop scores cannot be compared with each other.
- Is it a lab score or real-visitor data? The second is what Google looks at.
- Which page was tested? The home page is often the lightest; category and product pages tell you more.
- What exactly changed between the two tests, and were both run the same way?
What score is realistic for your site depends on the platform and the hosting; in our website speed optimisation work we say so after the first measurement, not before it.