chromium/third_party/blink/perf_tests/performance_apis/performance-now.html

<!DOCTYPE html>
<html>
<body>
<script src="../resources/runner.js"></script>
<script>
let results = [];
PerfTestRunner.measureTime({
  description: "Measures performance of performance.now() calls.",

  run: function() {
    for (var i = 0; i < 500000; ++i) {
      results.push(performance.now());
    }
  }
});
</script>
</body>
</html>