Try this :
var imageAddr = "http://www.testapplication.com/images/31120037-5mb.jpg";
var downloadSize = 4995374;
window.onload = function() {
var oProgress = document.getElementById("progress");
oProgress.innerHTML = "Loading the image, please wait...";
window.setTimeout(MeasureConnectionSpeed, 1);
};
function MeasureConnectionSpeed() {
var oProgress = document.getElementById("progress");
var startTime, endTime;
var download = new Image();
download.onload = function () {
endTime = (new Date()).getTime();
showResults();
}
download.onerror = function (err, msg) {
oProgress.innerHTML = "Invalid image, or error downloading";
}
startTime = (new Date()).getTime();
var cacheBuster = "?nnn=" + startTime;
download.src = imageAddr + cacheBuster;
function showResults() {
var duration = (endTime - startTime) / 1000;
var bitsLoaded = downloadSize * 8;
var speedBps = (bitsLoaded / duration).toFixed(2);
var speedKbps = (speedBps / 1024).toFixed(2);
var speedMbps = (speedKbps / 1024).toFixed(2);
oProgress.innerHTML = "Your connection speed is: <br />" +
speedBps + " bps<br />" +
speedKbps + " kbps<br />" +
speedMbps + " Mbps<br />";
}
}
the idea is load image with a known file size then in its onload event measure how much time passed until that event was triggered, and divide this time in the image file size.