If a browser does not use the Navigation timing API for page load timing (sometimes referred to as real user monitoring or RUM), the browser uses a cookie to determine the navigation start time. If a user navigates from a site without page load timing to a site with page load timing (for example, to the first page on your page load timing-enabled site), special considerations for navigation start time arise.
In this case Browser monitoring has all the data (queue time, app time, DOM content and rendering time) except network time. Navigation start time is required to compute network time. Rather than toss the data, the page load timing feature estimates the network time based on recently accumulated averages for your application, regionally. This way the page load timing feature provides a more accurate throughput measurement, and the rest of the data is retained.
This is especially important for sites with a high bounce rate (when users visit only one page). The more visitors you have from different regions, the more accurate the network estimate will be.
If a network estimate is not available for your application and for the region where the hit originated, the page load timing process reverts to a network estimate across all reporting apps for the region. Failing that, the data is tossed.
Note: For browsers that have the Navigation Timing API, this is not an issue, as the navigation start time can be determined without using a cookie.
Additional documentation resources include: