PageSpeed Insights Lab Data vs Field Data: A Frustrating Experience

Google's John Mueller answering a question about the difference between field and lab data in PageSpeed Insights.
SIA Team
November 22, 2021

Ideally, when considering Google’s PageSpeed Insights lab vs. field data, the two types of data should be closely reflective of each other. 

But first, we should ask: What’s field data, and what’s lab data?

To answer that, let’s consider our context: we’re using tools (as well as the experience of real users) to try to gauge something about our pages. 

In this case, a page’s loading time is one such metric we’re trying to measure, and we use tools such as Page Speed Insights, Lighthouse, GTMetrix, and others to try to accomplish this.

So, back to our question: field data is what users see, and lab data is what testing tools see. (That said, PageSpeed Insights can report field data.)

Recently, Google’s John Mueller addressed a frustrated webmaster. He was seeing large differences between the lab and field measurements, and despite his efforts to rectify these differences (to bring them closer together), he was not finding success. 

This was brought up around the 7-minute, 4-second mark of the latest English SEO Office-Hours. The video below is queued to that spot. 

PageSpeed Insights: Lab Data Vs Field Data: Which One Does Google Search Use?

For search, that’s the question. 

Upon hearing the participant’s situation, John’s response was: 

“I think there there’s one big aspect that plays a role there, which which is important in that, on the one hand, we have these lab-testing tools like PageSpeed Insights and Lighthouse and things like that where, essentially, you test it either from your browser with your connection or with some server. 

WebPageTest I think does similar things.  Some of the various other tools do that as well. That’s kind of the lab testing that you can do and that’s more something that is useful for incremental improvement.

For search, instead what we use is the field data, which is basically what users actually saw, and that’s the data that you see in Search Console, so essentially, what you’re probably seeing there is [that] users saw that something was slow…kind of on an aggregate level.”

So, when considering PageSpeed Insights’ lab data vs field data, just remember that, while both are important, in search field data is what’s considered. 

Source: Google Search Central YouTube channel