RAIL

RAIL, an acronym for Response, Animation, Idle and Load, is a performance model originated by the Google Chrome team in 2015, focused on user experience and performance within the browser. The performance mantra of RAIL is "Focus on the user; the end goal isn't to make your site perform fast on any specific device, it's to make users happy." There are 4 stages of interaction: page load, idle, response to input, and scrolling and animation. In acronym order, the main tenets are:

Response
Respond to users immediately, acknowledging any user input in 100ms or less.
Animation
In animating, render each frame in under 16ms, aiming for consistency and avoiding jank.
Idle
When using the main JavaScript thread, work in chunks for less than 50ms to free up the thread for user interactions.
Load
Deliver interactive content in under 1 second.

See also