PERF: Faster CategoricalIndex from categorical

Author: TomAugspurger

doc/source/whatsnew/v0.21.0.txt

@@ -415,6 +415,7 @@ Performance Improvements
 - Improved performance of instantiating :class:`SparseDataFrame` (:issue:`16773`)
 - :attr:`Series.dt` no longer performs frequency inference, yielding a large speedup when accessing the attribute (:issue:`17210`)
 - :attr:`Timestamp.microsecond` no longer re-computes on attribute access (:issue:`17331`)
+- Improved performance of the :class:`CategoricalIndex` for data that is already categorical dtype (:issue:`17513`)
 
 .. _whatsnew_0210.bug_fixes:
 

pandas/core/indexes/category.py

@@ -130,6 +130,9 @@ def _create_categorical(self, data, categories=None, ordered=None):
         -------
         Categorical
         """
+        if isinstance(data, ABCSeries) and is_categorical_dtype(data):
+            data = data.values
+
         if not isinstance(data, ABCCategorical):
             ordered = False if ordered is None else ordered
             from pandas.core.categorical import Categorical