The idea that a web page's first render should ideally complete in under one second, because users bounce rapidly after ~2s and most are lost beyond 10s. Even 100ms slowdowns change behavior (per a Google experiment) but over weeks. After subtracting DNS, TCP, TLS, HTML/CSS/JS downloads and ~100ms browser rendering, only ~300–400ms are left for the server to deliver the page. Setting and defending this time budget organizationally is presented as the single most important performance practice.