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chromium/third_party/blink/perf_tests/editing/page-down-with-many-lines.html 

<!doctype html>
<script src="../resources/runner.js"></script>
<textarea id="text" style="width:300px; height:300px" spellcheck="false"></textarea>
<script>
const kCount = 10;
const kLines = 20000
const kCharactersPerPage = 482;

text.textContent = (() => {
  const result = [];
  for (let count = 0; count < kLines; ++count)
    result.push(`${('00000' + count).slice(-5)} of brown foxes\n`);
  return result.join('');
})();
text.focus();

PerfTestRunner.measureRunsPerSecond({
  description: 'Measures performance of move-page-down on many lines',
  run: () => {
    const cursorIndex = text.value.length - 1 - kCharactersPerPage * kCount;
    text.setSelectionRange(cursorIndex, cursorIndex);
    for (let counter = 0; counter < kCount; ++counter)
      testRunner.execCommand("MovePageDown");
    },
});
</script>