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:
Respond to users immediately, acknowledging any user input in 100ms or less.
When animating, render each frame in under 16ms, aiming for consistency and avoiding jank.
Deliver interactive content in less than 1 second.