We run all our sites to the recommendations that are given by Google in the developers tools for user experience improvement.
So - putting it simply - Google Analytics measures how long it takes to load every script and file on the page. But this doesn't tell the whole story (a lot of those files are invisible to a user and don't affect user experience).
Google developer tools recommendations include 'lazy loading' or 'smart loading' (especially for mobile) to improve UX. This is when you front load the most important files - getting all the main content up in front of the user immediately, and taking more time over non essential files (like the ads or the FB button etc).