tmp-platform-swr/web/index.php

<?php
header('Cache-Control: s-max-age=10, stale-while-revalidate=20');
sleep(5);
print('This is a slow test. ' . date('Y-m-d H:i:s'));
?>
<ul>
  <li><a href="./slow.php">Slow</a></li>
  <li><a href="./slow-but-fast.php">Slow but fast</a></li>
</ul>
<p id="js-date"></p>
<script type="text/javascript">
  document.getElementById('js-date').innerText = new Date().toISOString();
</script>