Google Analytics has a number of powerful reports that automatically measure and report on page load times. However, it is also possible to track custom timing information to measure performance information specific to your site.
With User Timings, you can track and visualize user defined custom timings about websites. The report shows the execution speed or load time of any discrete hit, event, or user interaction that you want to track.
This can include measuring how quickly specific images and/or resources load, how long it takes for your site to respond to specific button clicks, timings for AJAX actions before and after onLoad event, etc. User timings will not alter your pageview count, hence, makes it the preferred method for tracking a variety of timings for actions in your site.
Source: Google Developers