Better illustrate the purpose

web/index.php

@@ -1,4 +1,9 @@
 <?php
+// First visit -> 5s, times match
+// Second visit -> instant, times don't match
+// … T+14 -> instant, times don't match
+// … T+15 (or 16) -> instant, times match (or almost)
+// Nothing for 30s -> 5s, times match
 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'));

web/slow-but-fast.php

@@ -1,5 +1,5 @@
 <?php
-header('Cache-Control: s-max-age=1, stale-while-revalidate=20');
+header('Cache-Control: s-max-age=1, stale-while-revalidate=99999999');
 sleep(5);
 print('This is a slow instant test. ' . date('Y-m-d H:i:s'));
 ?>