diff -u b/core/includes/file.inc b/core/includes/file.inc
--- b/core/includes/file.inc
+++ b/core/includes/file.inc
@@ -947,18 +947,12 @@
     $dir_has_slash = (substr($dir, -1) === '/');
   }
 
-  // By default, when there is no explicit 'nomask' option, do not check for
-  // files in common special-purpose directories. The folders here are front-end
-  // related and they have been added to avoid issues with Drupal recursive
-  // scanning. In this case, we added node_modules and bower_components. This
-  // also improves performance on frontend builds.
-  if (!isset($options['nomask'])) {
-    $ignore_directories = Settings::get('drupal_file_scan_ignore_directories', [
-      'node_modules',
-      'bower_components',
-    ]);
-    $options['nomask'] = '/^' . implode('|', $ignore_directories) . '$/';
-  }
+  // Allow directories specified in settings.php to be ignored. You can use this
+  // to not check for files in common special-purpose directories. For example,
+  // node_modules and bower_components. Ignoring irrelevant directories is a
+  // performance boost.
+  $ignore_directories = Settings::get('file_scan_ignore_directories', []);
+  $default_nomask = '/^' . implode('|', $ignore_directories) . '$/';
 
   $options['key'] = in_array($options['key'], array('uri', 'filename', 'name')) ? $options['key'] : 'uri';
   $files = array();
@@ -968,7 +962,10 @@
     if ($handle = @opendir($dir)) {
       while (FALSE !== ($filename = readdir($handle))) {
         // Skip this file if it matches the nomask or starts with a dot.
-        if ($filename[0] != '.' && !(isset($options['nomask']) && preg_match($options['nomask'], $filename))) {
+        if ($filename[0] != '.'
+          && !(isset($options['nomask']) && preg_match($options['nomask'], $filename))
+          && !(!empty($default_nomask) && preg_match($default_nomask, $filename))
+          ) {
           if ($depth == 0 && $dir_has_slash) {
             $uri = "$dir$filename";
           }
diff -u b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php
--- b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php
+++ b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterIterator.php
@@ -71,9 +71,6 @@
     'js',
     'misc',
     'templates',
-    // Deeply nested Node.js subdirectories.
-    'node_modules',
-    'bower_components',
     // Legacy subdirectories.
     'includes',
     // Test subdirectories.
@@ -90,6 +87,20 @@
   protected $acceptTests = FALSE;
 
   /**
+   * Construct a RecursiveExtensionFilterIterator.
+   *
+   * @param \RecursiveIterator $iterator
+   *   The iterator to filter.
+   * @param array $blacklist
+   *   (optional) Add to the blacklist of directories that should be filtered
+   *   out during the iteration.
+   */
+  public function __construct(\RecursiveIterator $iterator, array $blacklist = []) {
+    parent::__construct($iterator);
+    $this->blacklist += $blacklist;
+  }
+
+  /**
    * Controls whether test directories will be scanned.
    *
    * @param bool $flag
diff -u b/sites/default/default.settings.php b/sites/default/default.settings.php
--- b/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -706,15 +706,14 @@
 
 /**
  * The list of folders disabled for directory scanning.
- * As default values we added node_modules and bower_components
- * folders to avoid issues with Drupal recursive scanning.
  *
- * Uncomment the below strings to override default state.
+ * By default ignore node_modules and bower_components folders to avoid issues
+ * with Drupal recursive scanning.
  */
-# $settings['drupal_file_scan_ignore_directories'] = [
-#   'node_modules',
-#   'bower_components',
-# ];
+$settings['file_scan_ignore_directories'] = [
+  'node_modules',
+  'bower_components',
+];
 
 /**
  * Load local development override configuration, if available.
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -433,11 +433,17 @@ protected function scanDirectory($dir, $include_tests) {
     $flags |= \FilesystemIterator::CURRENT_AS_SELF;
     $directory_iterator = new \RecursiveDirectoryIterator($absolute_dir, $flags);
 
+    // Allow directories specified in settings.php to be ignored. You can use
+    // this to not check for files in common special-purpose directories. For
+    // example, node_modules and bower_components. Ignoring irrelevant
+    // directories is a performance boost.
+    $ignore_directories = Settings::get('file_scan_ignore_directories', []);
+
     // Filter the recursive scan to discover extensions only.
     // Important: Without a RecursiveFilterIterator, RecursiveDirectoryIterator
     // would recurse into the entire filesystem directory tree without any kind
     // of limitations.
-    $filter = new RecursiveExtensionFilterIterator($directory_iterator);
+    $filter = new RecursiveExtensionFilterIterator($directory_iterator, $ignore_directories);
     $filter->acceptTests($include_tests);
 
     // The actual recursive filesystem scan is only invoked by instantiating the