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chromium/third_party/blink/perf_tests/shadow_dom/imperative-api-appendchild.html 

<!DOCTYPE html>
<script src="../resources/runner.js"></script>
<div id="host">
</div>
<script>
  const host = document.querySelector("#host");
  const shadow_root = host.attachShadow({ mode: 'open', slotAssignment: 'manual' });
  const slot1 = document.createElement("slot");
  const child1 = document.createElement("child");
  slot1.assign(child1);
  shadow_root.appendChild(slot1);
  window.onload = function() {
    PerfTestRunner.measureTime({
      description: "Measure performance of appendChild in manual-slotting mode in shadow root.",
      run: function() {
        const start = PerfTestRunner.now();
        for (let i = 0; i < 100; i++) {
          host.appendChild(child1);
          child1.remove();
        }

        return PerfTestRunner.now() - start;
      }
    });
  }
</script>