LayoutTests/fast/shadow-dom/host-child-append-performance.html

<div id=host>This test passes if it doesn't time out</div>
<script>
if (window.testRunner)
    testRunner.dumpAsText();

const shadow = host.attachShadow({ mode: "open" });
shadow.innerHTML = `
    <slot></slot>
`;
for (i = 0; i < 10000; ++i)
    host.appendChild(document.createElement("div"));
</script>