Paul Irish and Paul Lewis describe a way for us to think about web performance: RAIL (Response, Animation, Idle and Load). It’s designed to break up each moment of a user’s experience into actions which can then be specifically optimized.
Very few of us have unlimited time to throw at optimization work, far from it, and we need criteria that help us decide what’s important to optimize (and what’s not!). When all is said is done, we want and need clear guidance on what “performant” means to our users, because that’s who we’re building for.
Working on web performance is a combination of obvious best practices (optimize assets), very tricky decisions (what can I defer loading on and what can’t I?), and nuanced choices (which animation technique is the most appropriate?).