var howManyTimes:Number = 3; //How many times per second  the download speed will be traced
var bytesLoaded:Number = 0; //don't change, necessary for calculation
var lastTime:int = 0; //don't change, necessary for calculation
var content:DisplayObject; //this variable will hold the content loaded

var loader:Loader = new Loader();
loader.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS, kbsPerSec);
loader.contentLoaderInfo.addEventListener(Event.COMPLETE, onComplete);
lastTime = getTimer();
loader.load(new URLRequest("heavy_image.jpg"));

//kbsPerSec listens to ProgressEvent.PROGRESS and traces the download speed "howManyTimes" times per second
function kbsPerSec(event:ProgressEvent):void
{
	var time:int = getTimer();
	if(time - lastTime >= (1000/howManyTimes))
	{
		var kiloBytes:Number = (event.bytesLoaded - bytesLoaded)/1000;
		var timeInSecs:Number = (time - lastTime)/1000;
		
		var kbsPerSecVal:Number = Math.floor(kiloBytes/timeInSecs);
		trace(kbsPerSecVal + " kbs/s");
		bytesLoaded = event.bytesLoaded;
		lastTime = getTimer();
	}
}

function onComplete(event:Event):void
{
	loader.contentLoaderInfo.removeEventListener(Event.COMPLETE, onComplete);
	loader.contentLoaderInfo.removeEventListener(ProgressEvent.PROGRESS, kbsPerSec);
	content = loader.content;
	loader = null;
	this.addChild(content);
}

A simple snippet showing how to display download speed (Kilobytes per second or kbs/s) in flash timeline script (AS3). You can use Flash "Simulate Download" option for testing.

How to measure download speed (Kilobytes per second) in AS3

ActionScript 3