First CPU Idle and Time to Interactive (TTI) help to pinpoint the time at which a page will respond in an acceptable amount of time.
Calibre works backwards from the time of document completion until it finds a point at which the browser is at rest. If a user were to interact with the page, there would be notable delay.
Calibre tracks the a number of interactivity metrics: First CPU Idle and Time to Interactive. These are great candidates for improving the user experience.
Each Calibre test has a timeout of 60 seconds, if a page does not become interactive within 60 seconds, the metric will not be recorded for the run.
For a more detailed look at Time to Interactive, please see our detailed blog post.