Measure how fast an image loads: mainly to test network speed

speed_test.js

/*
This example is the basis of a simple WordPress plugin 
which measures the download speed of a sample image
and then sends this information to the server for further action.
(e.g. writing to a database)
*/

function measureSpeed() {
    var duration = (endTime - startTime) / 1000;
    var bitsLoaded = downloadSize * 8;
    var speedBps = Math.round(bitsLoaded / duration);
    try{
        // add your functionality to define what you want to do with the
        // ajax response. e.g. set a cookie or whatever
    }catch(e){}
}

var startTime = (new Date()).getTime(), 
	endTime,
	imageAddr = 'assets/speed.png?n=' + Math.random(), // change to match the path of the sample image
	downloadSize = 1441, // file size of the sample image in bytes
	download = new Image();

download.src = imageAddr;

download.onload = function () {
    endTime = (new Date()).getTime();
    measureSpeed();
};