diff --git a/src/mistralai/utils/serializers.py b/src/mistralai/utils/serializers.py
index 378a14c..8468f9e 100644
--- a/src/mistralai/utils/serializers.py
+++ b/src/mistralai/utils/serializers.py
@@ -14,6 +14,10 @@
 
 from ..types.basemodel import BaseModel, Nullable, OptionalNullable, Unset
 
+_DECIMAL_UNSET_TYPES = (Decimal, Unset)
+
+_STR_INT_FLOAT_TYPES = (str, int, float)
+
 
 def serialize_decimal(as_str: bool):
     def serialize(d):
@@ -35,13 +39,21 @@ def validate_decimal(d):
     if d is None:
         return None
 
-    if isinstance(d, (Decimal, Unset)):
+    # Compare type directly for Decimal (bypasses isinstance overhead, but leaves Unset for legacy behavior)
+    if type(d) is Decimal or type(d) is Unset:
+        return d
+
+    # Only check for most common types first (fast-path), then do the expensive isinstance otherwise
+    if type(d) in _STR_INT_FLOAT_TYPES:
+        return Decimal(str(d))
+
+    if isinstance(d, _DECIMAL_UNSET_TYPES):
         return d
 
-    if not isinstance(d, (str, int, float)):
-        raise ValueError("Expected string, int or float")
+    if isinstance(d, _STR_INT_FLOAT_TYPES):
+        return Decimal(str(d))
 
-    return Decimal(str(d))
+    raise ValueError("Expected string, int or float")
 
 
 def serialize_float(as_str: bool):
@@ -178,7 +190,7 @@ 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
 
     for arg in get_args(field):