How can I monitor the performance of my XML3D application?

A nice way is to use the stats counter from: http://github.com/mrdoob/stats.js/

In the WebGL version, one can use the framedrawn event to update the counter, e.g:

var stats = new Stats(); 

stats.domElement.style.position='absolute';
stats.domElement.style.left='0px';
stats.domElement.style.top='0px'; 

var targetElement = ...
targetElement.style.position='realtive';
targetElement.appendChild(stats.domElement); 

var myxml3d = document.getElementById("myxml3d");
myxml3d.addEventListener("framedrawn", function(e) { stats.update(); }, false);

In the native version, one could use the “onframe” attribute to trigger the update, I didn’t test is yet. Both callbacks are in-official as they could be used to do timing attacks.

In Google Chrome you can also activate a FPS-Counter in chrome://flags/

Please be aware that this measures not just the rendering, but the overall performance of the webpage.