skip filtering & sorting if there's only one result

Closes #3351694

src/Plugin/Layout/LayoutOptions.php

@@ -540,6 +540,11 @@ class LayoutOptions extends LayoutDefault implements PluginFormInterface, Contai
    *   The sorted results.
    */
   protected function filterAndSortResultsByThemeHierarchy(array $results): array {
+    // If there's only one result, we can and should skip this step.
+    if (count($results) == 1) {
+      return $results;
+    }
+
     $default_theme_name = $this->themeHandler->getDefault();
     $themes = $this->themeHandler->listInfo();
     // Create a list which includes the current theme and all its base themes.