3 seconds for a page to load 74% OF MOBILE WEB USERS will leave a site if it takes longer than 5 seconds to load 1/3 of them will go to a competitor site
Webpagetest.org View them on : http://www.softwareishard.com/har/viewer/ Save - Export them as HAR (HTTP Archive) files by : • right click on network output and "Save as HAR with Content“ • Install add-on, Net-export Waterfalls
as possible with CSS • Use Data-uri • Compress them with tools as PNGGauntlet • Prefetch if sure you need them • Lazy-load them • Define their width & height (not responsive)