Index: src/infrastructure/markup/PhabricatorMarkupEngine.php
===================================================================
--- src/infrastructure/markup/PhabricatorMarkupEngine.php
+++ src/infrastructure/markup/PhabricatorMarkupEngine.php
@@ -202,13 +202,16 @@
     PhabricatorMarkupInterface $object,
     $field) {
 
-    $custom = array_merge(
-      self::loadCustomInlineRules(),
-      self::loadCustomBlockRules());
-
-    $custom = mpull($custom, 'getRuleVersion', null);
-    ksort($custom);
-    $custom = PhabricatorHash::digestForIndex(serialize($custom));
+    static $custom;
+    if ($custom === null) {
+      $custom = array_merge(
+        self::loadCustomInlineRules(),
+        self::loadCustomBlockRules());
+
+      $custom = mpull($custom, 'getRuleVersion', null);
+      ksort($custom);
+      $custom = PhabricatorHash::digestForIndex(serialize($custom));
+    }
 
     return $object->getMarkupFieldKey($field).'@'.$this->version.'@'.$custom;
   }