PERF/ENH: add fast astyping for Categorical

asv_bench/benchmarks/categoricals.py

@@ -67,6 +67,24 @@ def time_existing_series(self):
         pd.Categorical(self.series)
 
 
+class AsType:
+    def setup(self):
+        N = 10 ** 6
+
+        self.df = pd.DataFrame(
+            np.random.default_rng()
+            .choice(np.array(list("abcde")), 4 * N)
+            .reshape(N, 4),
+            columns=list("ABCD"),
+        )
+
+        for col in self.df.columns:
+            self.df[col] = self.df[col].astype("category")
+
+    def astype_unicode(self):
+        [self.df[col].astype("unicode") for col in self.df.columns]
+
+
 class Concat:
     def setup(self):
         N = 10 ** 5

doc/source/whatsnew/v1.2.0.rst

@@ -347,6 +347,7 @@ Performance improvements
 - Small performance decrease to :meth:`Rolling.min` and :meth:`Rolling.max` for fixed windows (:issue:`36567`)
 - Reduced peak memory usage in :meth:`DataFrame.to_pickle` when using ``protocol=5`` in python 3.8+ (:issue:`34244`)
 - Performance improvement in :class:`ExpandingGroupby` (:issue:`37064`)
+- Performance improvement in :meth:`DataFrame.astype` for :class:`Categorical` (:issue:`8628`)
 
 .. ---------------------------------------------------------------------------
 

pandas/core/internals/blocks.py

@@ -46,6 +46,7 @@
     is_re,
     is_re_compilable,
     is_sparse,
+    is_string_like_dtype,
     is_timedelta64_dtype,
     pandas_dtype,
 )
@@ -596,6 +597,17 @@ def astype(self, dtype, copy: bool = False, errors: str = "raise"):
 
             return self.make_block(Categorical(self.values, dtype=dtype))
 
+        elif (  # GH8628
+            is_categorical_dtype(self.values.dtype)
+            and not (is_object_dtype(dtype) or is_string_like_dtype(dtype))
+            and copy is True
+        ):
+            return self.make_block(
+                Categorical.from_codes(
+                    self.values.codes, categories=self.values.categories.astype(dtype)
+                )
+            )
+
         dtype = pandas_dtype(dtype)
 
         # astype processing