diff -u b/core/lib/Drupal/Core/Plugin/Discovery/CacheDecorator.php b/core/lib/Drupal/Core/Plugin/Discovery/CacheDecorator.php
--- b/core/lib/Drupal/Core/Plugin/Discovery/CacheDecorator.php
+++ b/core/lib/Drupal/Core/Plugin/Discovery/CacheDecorator.php
@@ -65,14 +65,21 @@
    * Implements Drupal\Component\Plugin\Discovery\DicoveryInterface::getDefinition().
    */
   public function getDefinition($plugin_id) {
-    if (isset($this->definitions[$plugin_id])) {
-      return $this->definitions[$plugin_id];
+    // Optimize for fast access to definitions if they are already in memory.
+    if (isset($this->definitions)) {
+      // Avoid using a ternary that would create a copy of the array.
+      if (isset($this->definitions[$plugin_id])) {
+        return $this->definitions[$plugin_id];
+      }
+      else {
+        return;
+      }
     }
 
-    if (!isset($this->definitions) && $definitions = $this->getDefinitions()) {
-      if (isset($definitions[$plugin_id])) {
-        return $definitions[$plugin_id];
-      }
+    $definitions = $this->getDefinitions();
+    // Avoid using a ternary that would create a copy of the array.
+    if (isset($definitions[$plugin_id])) {
+      return $definitions[$plugin_id];
     }
   }
 
@@ -80,6 +87,7 @@
    * Implements Drupal\Component\Plugin\Discovery\DicoveryInterface::getDefinitions().
    */
   public function getDefinitions() {
+    // Optimize for fast access to definitions if they are already in memory.
     if (isset($this->definitions)) {
       return $this->definitions;
     }