diff --git a/src/mistralai/models/imageurl.py b/src/mistralai/models/imageurl.py
index 6f077b6..605a730 100644
--- a/src/mistralai/models/imageurl.py
+++ b/src/mistralai/models/imageurl.py
@@ -18,29 +18,32 @@ class ImageURL(BaseModel):
 
     @model_serializer(mode="wrap")
     def serialize_model(self, handler):
-        optional_fields = ["detail"]
-        nullable_fields = ["detail"]
-        null_default_fields = []
+        optional_fields = {"detail"}
+        nullable_fields = {"detail"}
+        null_default_fields = set()
 
         serialized = handler(self)
 
         m = {}
 
-        for n, f in type(self).model_fields.items():
+        # Cache fields_set for faster lookup, since intersection is O(n)
+        fields_set = self.__pydantic_fields_set__
+
+        # Use .items() directly for iterating, and avoid repeated lookups
+        model_fields = type(self).model_fields
+        for n, f in model_fields.items():
             k = f.alias or n
             val = serialized.get(k)
-            serialized.pop(k, None)
 
             optional_nullable = k in optional_fields and k in nullable_fields
-            is_set = (
-                self.__pydantic_fields_set__.intersection({n})
-                or k in null_default_fields
-            )  # pylint: disable=no-member
+            is_set = n in fields_set or k in null_default_fields  # pylint: disable=no-member
 
+            # Avoid using pop here as we never use serialized again and pop causes unnecessary internal work.
+            # Fast branch ordering for common cases (most keys are not UNSET_SENTINEL).
             if val is not None and val != UNSET_SENTINEL:
                 m[k] = val
             elif val != UNSET_SENTINEL and (
-                not k in optional_fields or (optional_nullable and is_set)
+                k not in optional_fields or (optional_nullable and is_set)
             ):
                 m[k] = val