diff --git a/CHANGELOG.md b/CHANGELOG.md
index 803dde9..b029c14 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 * perf: Add caching for `TransformerRegistry`.
 * perf: Optimize `guessTypeFromVariable`
 * perf: Optimize `Context` & `ObjectCache`
+* perf: Change 'simpletypes' cache to array.
 
 ## 0.5.14
 
diff --git a/src/TypeResolver/CachingTypeResolver.php b/src/TypeResolver/CachingTypeResolver.php
index 7a82bbb..5240da0 100644
--- a/src/TypeResolver/CachingTypeResolver.php
+++ b/src/TypeResolver/CachingTypeResolver.php
@@ -24,9 +24,6 @@ public function __construct(
         /** @psalm-suppress PropertyTypeCoercion */
         $this->typeStringCache = new \WeakMap();
 
-        /** @psalm-suppress PropertyTypeCoercion */
-        $this->simpleTypesCache = new \WeakMap();
-
         /** @psalm-suppress PropertyTypeCoercion */
         $this->isSimpleTypeCache = new \WeakMap();
     }
@@ -60,9 +57,9 @@ public function getTypeString(Type|MixedType $type): string
     // can be expensive in a loop. we cache using a weakmap
 
     /**
-     * @var \WeakMap<Type,array<array-key,MixedType|Type>>
+     * @var array<string,array<array-key,MixedType|Type>>
      */
-    private \WeakMap $simpleTypesCache;
+    private array $simpleTypesCache = [];
 
     public function getSimpleTypes(Type|MixedType $type): array
     {
@@ -70,13 +67,15 @@ public function getSimpleTypes(Type|MixedType $type): array
             return [$type];
         }
 
-        if ($result = $this->simpleTypesCache[$type] ?? null) {
+        $key = md5(serialize($type));
+
+        if ($result = $this->simpleTypesCache[$key] ?? null) {
             return $result;
         }
 
         $simpleTypes = $this->decorated->getSimpleTypes($type);
 
-        $this->simpleTypesCache->offsetSet($type, $simpleTypes);
+        $this->simpleTypesCache[$key] = $simpleTypes;
 
         return $simpleTypes;
     }