User Timing API
Web developers need the ability to assess and understand the performance characteristics of their applications. While JavaScript [ECMA262] provides a mechanism to measure application latency (retrieving the current timestamp from the Date.now() method), the precision of this timestamp varies between user agents.
This document defines the PerformanceMark and PerformanceMeasure interfaces, and extensions to the Performance interface, which expose a high precision timestamp to developers so they can better measure the performance of their applications.
User Timing
For the simplest view, turn on the console and watch for console.log() in the JavaScript window, and ignore all the code that relates to document.createDocumentFragment().