The Paint Timing API provides perceived performance data that allows you to know exactly how long a user waits, after navigating to your page, before seeing any of the page content at all (which may not yet be the main content) first starting to appear. More specifically, it allows you to know exactly when, during page load, first paint and first contentful paint occur.
The Largest Contentful Paint API is a closely-related API that allows you to know exactly how long a user waits before they can see the main content of your page in the browser viewport — in contrast to just knowing when the user can see what may be a loading indicator or background image or other such initial/early-loading content (which is what the Paint Timing API provides).