PageSpeed Insights provides both lab and field data about a page. Lab data is useful for debugging performance issues, as it is collected in a controlled environment. However, it may not capture real-world bottlenecks. Field data, on the other hand, is useful for capturing true, real-world user experiences – but has a more limited set of metrics. See How To Think About Speed Tools for more information on the 2 types of data.
Field Data – This section shows a rolling 30 day report of the page using the Chrome User Experience Report. The field data is basically a historical report about how a particular URL has performed over time, and represents anonymized performance data from users in the real-world on a variety of devices and network conditions.
PageSpeed Insights classifies field data into 3 buckets, describing experiences as fast, average, or slow. PSI sets the following thresholds for fast / average / slow, based on analysis of the CrUX dataset:
First Contentful Paint (FCP) marks the point, immediately after navigation, when the browser renders the first bit of content from the DOM. This is an important milestone for users because it provides feedback that the page is actually loading.
Origin Summary shows a developer the speed of all the pages that are served from this page compared to other pages in the Chrome User Experience Report over the last 30 days.