Using MutationObserver on http://browserbench.org/Speedometer/

content.js

var addedNodesCount = 0;
var removedNodesCount = 0;
var observer = new MutationObserver(function(mutations) {
    var i = mutations.length;
    while ( i-- ) {
        var mutation = mutations[i];
        addedNodesCount += mutation.addedNodes.length;
        removedNodesCount += mutation.removedNodes.length;
    }
    console.log('addedNodesCount=' + addedNodesCount, 'removedNodesCount=' + removedNodesCount);
});
observer.observe(document.documentElement, {
    childList: true,
    subtree: true
});

gistfile1.txt

...
content.js:10 addedNodesCount=76
content.js:10 addedNodesCount=78
content.js:10 addedNodesCount=88
content.js:10 addedNodesCount=658 removedNodesCount=516
content.js:10 addedNodesCount=10590
content.js:10 addedNodesCount=659 removedNodesCount=517
content.js:10 addedNodesCount=31190
content.js:10 addedNodesCount=660 removedNodesCount=518
content.js:10 addedNodesCount=31194
content.js:10 addedNodesCount=661 removedNodesCount=519
content.js:10 addedNodesCount=120
content.js:10 addedNodesCount=121
content.js:10 addedNodesCount=123
content.js:10 addedNodesCount=124
...

manifest.json

{
    "manifest_version": 2,
    "name": "webext-mutation-observer",
    "version": "1.0",
    "content_scripts": [
        {
            "matches": ["http://browserbench.org/*"],
            "js": ["content.js"],
            "run_at": "document_start",
            "all_frames": true
        }
    ],
    "permissions": [
        "http://browserbench.org/*"
    ]
}