<body>
<script type="text/javascript">var canvas = document.createElement('canvas');
canvas.setAttribute('width',32);
canvas.setAttribute('height',32);
var div = document.createElement('div');
document.body.appendChild(canvas);
document.body.appendChild(div);
var ctx = canvas.getContext('2d'),
imageData = ctx.createImageData(32,32),
data = imageData.data,
l = 1024* 4;
(function randomDraw(){
var i = l, time = Date.now();
while(i--) data[i] = Math.floor(Math.random()*256);
ctx.putImageData(imageData,0,0);
div.innerHTML = 'Скорость отрисовки: ~' + (-time + (time = Date.now())) + 'ms';
setTimeout(randomDraw,50)
}())</script>
</body>