What is the 3 second rule in web design?

It is the idea that you have about three seconds to earn attention: the page should load quickly and communicate value immediately. In practice, that means fast performance (especially on mobile), a clear headline, and visible proof/CTA above the fold. If users are staring at a blank screen or confused hero section, they bounce. Treat it as a design and engineering requirement: optimize media, reduce JavaScript weight, and build a layout that answers “what is this and why should I care?” instantly.