The 1st and 3rd Party Composition widget gives you a breakdown of the performance, size and amount of the content on your site. This widget is available for both Real User and Synthetic Data.
Note: 1st Party content is defined by the "First Party Domains" list in the site settings. All else is marked as 3rd Party.
Domain Activity - The sum of the time content from a particular domain spent loading. In this context, it is the sum of time each of 1st and 3rd party domains spent loading, on average over the Dashboard Time Period.
File Size - The sum of the file sizes from each of the 1st and 3rd Party content, on average over the Dashboard Time Period.
Element Count / Page - The number of elements of each of the 1st and 3rd party content, on average over the Dashboard Time Period.