diff --git a/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterCallback.php b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterCallback.php
new file mode 100644
index 0000000..a834bd9
--- /dev/null
+++ b/core/lib/Drupal/Core/Extension/Discovery/RecursiveExtensionFilterCallback.php
@@ -0,0 +1,159 @@
+<?php
+
+namespace Drupal\Core\Extension\Discovery;
+
+/**
+ * Filters a RecursiveDirectoryIterator to discover extensions.
+ *
+ * To ensure the best possible performance for extension discovery, this
+ * filter implementation hard-codes a range of assumptions about directories
+ * in which Drupal extensions may appear and in which not. Every unnecessary
+ * subdirectory tree recursion is avoided.
+ *
+ * The list of globally ignored directory names is defined in the
+ * RecursiveExtensionFilterIterator::$blacklist property.
+ *
+ * In addition, all 'config' directories are skipped, unless the directory path
+ * ends with 'modules/config', so as to still find the config module provided by
+ * Drupal core and still allow that module to be overridden with a custom config
+ * module.
+ *
+ * Lastly, ExtensionDiscovery instructs this filter to additionally skip all
+ * 'tests' directories at regular runtime, since just with Drupal core only, the
+ * discovery process yields 4x more extensions when tests are not ignored.
+ *
+ * @see ExtensionDiscovery::scan()
+ * @see ExtensionDiscovery::scanDirectory()
+ *
+ * @internal
+ */
+class RecursiveExtensionFilterCallback {
+
+  /**
+   * List of base extension type directory names to scan.
+   *
+   * Only these directory names are considered when starting a filesystem
+   * recursion in a search path.
+   *
+   * @var array
+   */
+  protected $whitelist = array(
+    'profiles',
+    'modules',
+    'themes',
+  );
+
+  /**
+   * List of directory names to skip when recursing.
+   *
+   * These directories are globally ignored in the recursive filesystem scan;
+   * i.e., extensions (of all types) are not able to use any of these names,
+   * because their directory names will be skipped.
+   *
+   * @var array
+   */
+  protected $blacklist = array(
+    // Object-oriented code subdirectories.
+    'src',
+    'lib',
+    'vendor',
+    // Front-end.
+    'assets',
+    'css',
+    'files',
+    'images',
+    'js',
+    'misc',
+    'templates',
+    // Legacy subdirectories.
+    'includes',
+    // Test subdirectories.
+    'fixtures',
+    // @todo ./tests/Drupal should be ./tests/src/Drupal
+    'Drupal',
+  );
+
+  /**
+   * Whether to include test directories when recursing.
+   *
+   * @var bool
+   */
+  protected $acceptTests = FALSE;
+
+  /**
+   * Construct a RecursiveExtensionFilterIterator.
+   *
+   * @param array $blacklist
+   *   (optional) Add to the blacklist of directories that should be filtered
+   *   out during the iteration.
+   * @param bool $accept_tests
+   *   Pass FALSE to skip all test directories in the discovery. If TRUE,
+   *   extensions in test directories will be discovered and only the global
+   *   directory blacklist in RecursiveExtensionFilterIterator::$blacklist is
+   *   applied.
+   */
+  public function __construct(array $blacklist = [], $accept_tests = FALSE) {
+    $this->blacklist = array_merge($this->blacklist, $blacklist);
+
+    $this->acceptTests($accept_tests);
+  }
+
+  /**
+   * Controls whether test directories will be scanned.
+   *
+   * @param bool $flag
+   *   Pass FALSE to skip all test directories in the discovery. If TRUE,
+   *   extensions in test directories will be discovered and only the global
+   *   directory blacklist in RecursiveExtensionFilterIterator::$blacklist is
+   *   applied.
+   */
+  public function acceptTests($flag = FALSE) {
+    $this->acceptTests = $flag;
+    if (!$this->acceptTests) {
+      $this->blacklist[] = 'tests';
+    }
+  }
+
+  public function accept(\SplFileInfo $current, $key, \RecursiveDirectoryIterator $iterator) {
+    $name = $current->getFilename();
+    // FilesystemIterator::SKIP_DOTS only skips '.' and '..', but not hidden
+    // directories (like '.git').
+    if ($name[0] == '.') {
+      return FALSE;
+    }
+
+    // Allow recursion
+    if ($iterator->hasChildren()) {
+      return TRUE;
+    }
+
+    if ($current->isDir()) {
+      // If this is a subdirectory of a base search path, only recurse into the
+      // fixed list of expected extension type directory names. Required for
+      // scanning the top-level/root directory; without this condition, we would
+      // recurse into the whole filesystem tree that possibly contains other
+      // files aside from Drupal.
+      if ($current->getSubPath() == '') {
+        return in_array($name, $this->whitelist, TRUE);
+      }
+      // 'config' directories are special-cased here, because every extension
+      // contains one. However, those default configuration directories cannot
+      // contain extensions. The directory name cannot be globally skipped,
+      // because core happens to have a directory of an actual module that is
+      // named 'config'. By explicitly testing for that case, we can skip all
+      // other config directories, and at the same time, still allow the core
+      // config module to be overridden/replaced in a profile/site directory
+      // (whereas it must be located directly in a modules directory).
+      if ($name == 'config') {
+        return substr($current->getPathname(), -14) == 'modules/config';
+      }
+      // Accept the directory unless the name is blacklisted.
+      return !in_array($name, $this->blacklist, TRUE);
+    }
+    else {
+      // Only accept extension info files.
+      return substr($name, -9) == '.info.yml';
+    }
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
index 0d9283c..dcd9804 100644
--- a/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
+++ b/core/lib/Drupal/Core/Extension/ExtensionDiscovery.php
@@ -4,8 +4,9 @@
 
 use Drupal\Component\FileCache\FileCacheFactory;
 use Drupal\Core\DrupalKernel;
-use Drupal\Core\Extension\Discovery\RecursiveExtensionFilterIterator;
+use Drupal\Core\Extension\Discovery\RecursiveExtensionFilterCallback;
 use Drupal\Core\Site\Settings;
+use RecursiveCallbackFilterIterator;
 use Symfony\Component\HttpFoundation\Request;
 
 /**
@@ -401,7 +402,7 @@ protected function process(array $all_files) {
    *   are associative arrays of \Drupal\Core\Extension\Extension objects, keyed
    *   by absolute path name.
    *
-   * @see \Drupal\Core\Extension\Discovery\RecursiveExtensionFilterIterator
+   * @see \Drupal\Core\Extension\Discovery\RecursiveExtensionFilterCallback
    */
   protected function scanDirectory($dir, $include_tests) {
     $files = array();
@@ -437,8 +438,8 @@ protected function scanDirectory($dir, $include_tests) {
     // Important: Without a RecursiveFilterIterator, RecursiveDirectoryIterator
     // would recurse into the entire filesystem directory tree without any kind
     // of limitations.
-    $filter = new RecursiveExtensionFilterIterator($directory_iterator, $ignore_directories);
-    $filter->acceptTests($include_tests);
+    $callback = new RecursiveExtensionFilterCallback($ignore_directories, $include_tests);
+    $filter = new RecursiveCallbackFilterIterator($directory_iterator, [$callback, 'accept']);
 
     // The actual recursive filesystem scan is only invoked by instantiating the
     // RecursiveIteratorIterator.