diff --git a/fastapi/encoders.py b/fastapi/encoders.py
index 851fdc3ee..0d46c059b 100644
--- a/fastapi/encoders.py
+++ b/fastapi/encoders.py
@@ -236,9 +236,12 @@ def encode_value(
     sqlalchemy_safe: bool = True,
 ) -> Any:
     if custom_encoder:
-        if type(obj) in custom_encoder:
-            return custom_encoder[type(obj)](obj)
+        # fastpath for exact class match
+        obj_type = type(obj)
+        if obj_type in custom_encoder:
+            return custom_encoder[obj_type](obj)
 
+        # fallback to isinstance which uses MRO
         for encoder_type, encoder_instance in custom_encoder.items():
             if isinstance(obj, encoder_type):
                 return encoder_instance(obj)