core/lib/Drupal/Core/Cache/Cache.php | 44 +++++++++++++++++++++++-------------
 1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/core/lib/Drupal/Core/Cache/Cache.php b/core/lib/Drupal/Core/Cache/Cache.php
index 6581546..e73aee6 100644
--- a/core/lib/Drupal/Core/Cache/Cache.php
+++ b/core/lib/Drupal/Core/Cache/Cache.php
@@ -22,12 +22,11 @@ class Cache {
   const PERMANENT = CacheBackendInterface::CACHE_PERMANENT;
 
   /**
+   * The set of valid cache context tokens.
    *
-   * Contains the valid cache contexts, pulled from the container.
-   *
-   * @var \Drupal\Core\Cache\CacheContextInterface[]
+   * @var string[]
    */
-  protected static $validCacheContexts;
+  protected static $validContextTokens;
 
   /**
    * Merges arrays of cache contexts and removes duplicates.
@@ -69,22 +68,35 @@ public static function validateContexts(array $context_tokens) {
 
     // The set of existing (thus valid) cache contexts is stored on the
     // container; it's safe to statically cache this because it cannot change
-    // during the request.
-    if (!isset(static::$validCacheContexts)) {
-      static::$validCacheContexts = \Drupal::getContainer()->getParameter('cache_contexts');
+    // during the request. Initialize the set of valid context tokens with this.
+    if (!isset(static::$validContextTokens)) {
+      static::$validContextTokens = \Drupal::getContainer()->getParameter('cache_contexts');
     }
 
-    foreach ($context_tokens as $value) {
-      if (!is_string($value)) {
-        throw new \LogicException('Cache contexts must be strings, ' . gettype($value) . ' given.');
+    $valid = array_flip(static::$validContextTokens);
+    foreach ($context_tokens as $context_token) {
+      if (!is_string($context_token)) {
+        throw new \LogicException('Cache contexts must be strings, ' . gettype($context_token) . ' given.');
+      }
+
+      if (isset($valid[$context_token])) {
+        continue;
+      }
+
+      // If it's a valid context token, then the ID must be stored in the set
+      // of valid context tokens (since we initialized it with the list of cache
+      // context IDs using the container). In case of an invalid context token,
+      // throw an exception, otherwise cache it, including the parameter, to
+      // minimize the amount of work in future ::validateContexts() calls.
+      $context_id = $context_token;
+      if (strpos($context_id, ':') !== FALSE) {
+        $context_id = substr($context_id, 0, strpos($context_id, ':'));
       }
-      // If a cache context token has a parameter, drop it to get the cache
-      // context ID.
-      if (strpos($value, ':') !== FALSE) {
-        $value = substr($value, 0, strpos($value, ':'));
+      if (isset($valid[$context_id])) {
+        static::$validContextTokens[] = $context_token;
       }
-      if (!in_array($value, static::$validCacheContexts)) {
-        throw new \LogicException('"' . $value. '" is not a valid cache context ID.');
+      else {
+        throw new \LogicException('"' . $context_id . '" is not a valid cache context ID.');
       }
     }
   }