chromium/third_party/blink/perf_tests/bindings/serialize-map.html

<!DOCTYPE html>
<body>
<script src="../resources/runner.js"></script>
<script>

var length = 200000;
var obj = {};  // obj does not have a 'length' property.
for (var i = 0; i < length; i++)
    obj['key' + i] = i;
var worker = new Worker('resources/worker.js');
worker.onmessage = function(event) {
    console.log("received");
};

PerfTestRunner.measureTime({
    description: "Measures performance of serializing a large map object.",
    run: function() {
        worker.postMessage(obj);
    }
});
</script>
</body>