window.performance

resourceTiming.js

// window.performance
function resourceTiming() 
{
   var resourceList = window.performance.getEntriesByType("resource");
   for (i = 0; i < resourceList.length; i++)
   {
      if (resourceList[i].initiatorType == "img") 
      {
         alert("End to end resource fetch: "+ (resourceList[i].responseEnd - resourceList[i].startTime));
      }
   }
}
resourceTiming();