Interactivity Metrics

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.

Why is monitoring Time to Interactive important?

Time to Interactive is influenced by JavaScript Main Thread activity. Pages that have a large amount of scripting will delay the time to interaction. Scripting performance varies greatly from device to device—slower CPUs spend far more time parsing and compiling, than faster ones.

How are these metrics calculated?

Calibre observes the point at which the main thread activity has no major JavaScript activity. The timeline below illustrates JavaScript activity on the main thread.

Major activity is categorised as JavaScript tasks that take longer than 50ms.

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.