diff --git a/src/mistralai/utils/serializers.py b/src/mistralai/utils/serializers.py
index 378a14c..449a6c6 100644
--- a/src/mistralai/utils/serializers.py
+++ b/src/mistralai/utils/serializers.py
@@ -157,20 +157,31 @@ def marshal_json(val, typ):
     if is_nullable(typ) and val is None:
         return "null"
 
-    marshaller = create_model(
-        "Marshaller",
-        body=(typ, ...),
-        __config__=ConfigDict(populate_by_name=True, arbitrary_types_allowed=True),
-    )
+    # Cache created model classes for (typ,) to avoid expensive recreation
+    if not hasattr(marshal_json, "_marshaller_cache"):
+        marshal_json._marshaller_cache = {}
+    marshaller_cache = marshal_json._marshaller_cache
+
+    if typ in marshaller_cache:
+        marshaller = marshaller_cache[typ]
+    else:
+        marshaller = create_model(
+            "Marshaller",
+            body=(typ, ...),
+            __config__=ConfigDict(populate_by_name=True, arbitrary_types_allowed=True),
+        )
+        marshaller_cache[typ] = marshaller
 
     m = marshaller(body=val)
 
+    # This produces a dict with a single key "body"
     d = m.model_dump(by_alias=True, mode="json", exclude_none=True)
 
-    if len(d) == 0:
+    if not d:
         return ""
 
-    return json.dumps(d[next(iter(d))], separators=(",", ":"))
+    # Direct access instead of next(iter(d))
+    return json.dumps(d["body"], separators=(",", ":"))
 
 
 def is_nullable(field):
@@ -178,11 +189,13 @@ def is_nullable(field):
     if origin is Nullable or origin is OptionalNullable:
         return True
 
-    if not origin is Union or type(None) not in get_args(field):
+    if origin is not Union or type(None) not in get_args(field):
         return False
 
+    # Only call get_origin(arg) once per arg
     for arg in get_args(field):
-        if get_origin(arg) is Nullable or get_origin(arg) is OptionalNullable:
+        arg_origin = get_origin(arg)
+        if arg_origin is Nullable or arg_origin is OptionalNullable:
             return True
 
     return False