PERF/ENH: add fast astyping for categorical input

Author: arw2019

pandas/core/generic.py

@@ -66,6 +66,7 @@
     ensure_str,
     is_bool,
     is_bool_dtype,
+    is_categorical,
     is_datetime64_any_dtype,
     is_datetime64tz_dtype,
     is_dict_like,
@@ -5794,6 +5795,12 @@ def astype(
                 for i in range(len(self.columns))
             ]
 
+        elif is_categorical(self):
+            new_data = pd.Categorical.from_codes(
+                self.cat.codes, categories=self.cat.categories.astype(dtype)
+            )
+            return self._constructor(new_data).__finalize__(self, method="astype")
+
         else:
             # else, only a single dtype is given
             new_data = self._mgr.astype(dtype=dtype, copy=copy, errors=errors)