Try changing order of statements for performance

Author: kinow

pandas/core/internals/blocks.py

@@ -739,12 +739,12 @@ def to_native_types(self, slicer=None, na_rep='nan', quoting=None,
         mask = isna(values)
 
         if not self.is_object and not quoting:
-            itemsize = writers.max_len_string(na_rep)
-            values = values.astype("<U{size}".format(size=itemsize))
+            values[mask] = na_rep
+            values = values.astype(str)
         else:
             values = np.array(values, dtype='object')
+            values[mask] = na_rep
 
-        values[mask] = na_rep
         return values
 
     # block actions ####