When a user visits a page, the time it takes for the browser to render the page usable is recorded in the domInteractive event.
To calculate the mobile page loading time, Google uses the Navigation Timing API to get the domInteractive timings for a sample of the traffic coming to your pages with AdSense ads, then take the median of the samples.
What is Navigation Timing API?
The Navigation Timing API provides data that can be used to measure the performance of a web site.
You can use the Navigation Timing API to gather performance data on the client side.
We can can then transmit that data to a server using XMLHttpRequest or other techniques.
Navigation Timing API lets you measure data that was previously difficult to obtain, such as
➛ the amount of time needed to unload the previous page,
➛ how long domain lookups take,
➛ the total time spent executing the window’s load handler and more.
The window.performance property returns a Performance object. The Navigation Timing API adds two properties to it: timing and navigation.
It provides methods and properties to store and retrieve metrics regarding the browser’s document navigation events.
For example, this interface can be used to determine how much time it takes to load or unload a document.
const perfData = window.performance.timing; const pageLoadTime = perfData.loadEventEnd - perfData.navigationStart;
This subtracts the time at which navigation began (navigationStart) from the time at which the load event handler returns (loadEventEnd). This gives us the perceived page load time.
Google Support and MDN