diff --git a/core/modules/big_pipe/js/big_pipe.es6.js b/core/modules/big_pipe/js/big_pipe.es6.js
index 647f38f022..54ae253465 100644
--- a/core/modules/big_pipe/js/big_pipe.es6.js
+++ b/core/modules/big_pipe/js/big_pipe.es6.js
@@ -3,7 +3,7 @@
  * Renders BigPipe placeholders using Drupal's Ajax system.
  */
 
-(function (Drupal, drupalSettings) {
+(function (window, Drupal, drupalSettings) {
   const placeholderIdAttr = 'data-big-pipe-replacement-for-placeholder-with-id';
   const replacementsSelector = `script[${placeholderIdAttr}]`;
 
@@ -94,9 +94,15 @@
 
   // As soon as the document is loaded, no more replacements will be added.
   // Immediately fetch and process all pending mutations and stop the observer.
-  window.addEventListener('DOMContentLoaded', () => {
-    const mutations = observer.takeRecords();
-    observer.disconnect();
-    processMutations(mutations);
-  });
-})(Drupal, drupalSettings);
+  window.Drupal.bigPipeProcessMutationsCallback =
+    drupalSettings.processMutationsCallback ||
+    function processMutationsCallback() {
+      const mutations = observer.takeRecords();
+      observer.disconnect();
+      processMutations(mutations);
+    };
+  window.addEventListener(
+    'DOMContentLoaded',
+    window.Drupal.bigPipeProcessMutationsCallback,
+  );
+})(window, Drupal, drupalSettings);
diff --git a/core/modules/big_pipe/js/big_pipe.js b/core/modules/big_pipe/js/big_pipe.js
index 34a21c6b4e..db22ee99e8 100644
--- a/core/modules/big_pipe/js/big_pipe.js
+++ b/core/modules/big_pipe/js/big_pipe.js
@@ -5,7 +5,7 @@
 * @preserve
 **/
 
-(function (Drupal, drupalSettings) {
+(function (window, Drupal, drupalSettings) {
   var placeholderIdAttr = 'data-big-pipe-replacement-for-placeholder-with-id';
   var replacementsSelector = "script[".concat(placeholderIdAttr, "]");
 
@@ -60,9 +60,12 @@
   observer.observe(document.body, {
     childList: true
   });
-  window.addEventListener('DOMContentLoaded', function () {
+
+  window.Drupal.bigPipeProcessMutationsCallback = drupalSettings.processMutationsCallback || function processMutationsCallback() {
     var mutations = observer.takeRecords();
     observer.disconnect();
     processMutations(mutations);
-  });
-})(Drupal, drupalSettings);
\ No newline at end of file
+  };
+
+  window.addEventListener('DOMContentLoaded', window.Drupal.bigPipeProcessMutationsCallback);
+})(window, Drupal, drupalSettings);
\ No newline at end of file