Other tries

web/slow-but-fast.php

@@ -0,0 +1,9 @@
+<?php
+header('Cache-Control: s-max-age=1, stale-while-revalidate=20');
+sleep(5);
+print('This is a slow instant test. ' . date('Y-m-d H:i:s'));
+?>
+<p id="js-date"></p>
+<script type="text/javascript">
+  document.getElementById('js-date').innerText = new Date().toISOString();
+</script>