Image optimization for the web
Images are almost always the heaviest thing on a web page. A single hero photo can weigh more than all the HTML, CSS and JavaScript of a site put together, and every extra kilobyte delays the moment a visitor finally sees something useful. Search engines notice it, and users on mobile data notice it faster. The encouraging part is that image delivery is one of the few areas where the fixes are well documented, mostly automatable, and visible in the metrics almost immediately.