diff --git a/asv_bench/benchmarks/categoricals.py b/asv_bench/benchmarks/categoricals.py
index 5464e7cba22c3..41460eaf47699 100644
--- a/asv_bench/benchmarks/categoricals.py
+++ b/asv_bench/benchmarks/categoricals.py
@@ -193,3 +193,53 @@ def time_categorical_series_is_monotonic_increasing(self):
 
     def time_categorical_series_is_monotonic_decreasing(self):
         self.s.is_monotonic_decreasing
+
+
+class Contains(object):
+
+    params = ([
+        "b",  # in array
+        "d",  # in categories but not in codes
+        "z",  # nowhere
+        np.nan,
+    ],
+        [True, False],
+    )
+    param_names = ["value", "has_nan"]
+
+    def setup(self, value, has_nan):
+        n = 1 * 10 ** 4
+        obj_values = list("a" * n + "b" * n + "c" * n)
+        if has_nan:
+            obj_values = [np.nan] + obj_values[:-2] + [np.nan]
+
+        self.ci = pd.CategoricalIndex(obj_values, categories=list("abcd"))
+        self.cat = pd.Categorical(obj_values, categories=list("abcd"))
+
+    def time_contains_index(self, value, has_nan):
+        value in self.ci
+
+    def time_cat_isin(self, value, has_nan):
+        value in self.cat
+
+
+class Indexing(object):
+
+    params = (["a", "c"], [True, False])
+    param_names = ["value", "has_nan"]
+
+    def setup(self, value, has_nan):
+        n = 1 * 10 ** 4
+        obj_values = list("a" * n + "b" * n + "c" * n)
+        if has_nan:
+            obj_values = [np.nan] + obj_values[:-2] + [np.nan]
+
+        ci = pd.CategoricalIndex(obj_values, categories=list("abcd"))
+        self.df = pd.DataFrame(dict(A=range(n * 3)), index=ci)
+        self.ser = pd.Series(range(n * 3), index=ci)
+
+    def time_loc_df(self, value, has_nan):
+        self.df.loc[value]
+
+    def time_loc_ser(self, value, has_nan):
+        self.ser.loc[value]
diff --git a/doc/source/whatsnew/v0.24.0.txt b/doc/source/whatsnew/v0.24.0.txt
index c69de149a0f35..4158edf683c89 100644
--- a/doc/source/whatsnew/v0.24.0.txt
+++ b/doc/source/whatsnew/v0.24.0.txt
@@ -64,7 +64,7 @@ Performance Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 - Improved performance of :func:`Series.describe` in case of numeric dtpyes (:issue:`21274`)
--
+- Improved performance of indexing on a Series/DataFrame with a ``CategoricalIndex`` (:issue:`21022`)
 
 .. _whatsnew_0240.docs:
 
@@ -83,7 +83,7 @@ Bug Fixes
 Categorical
 ^^^^^^^^^^^
 
--
+- Fixed an issue where membership checks on ``CategoricalIndex`` with interval values may return false positive (:issue:`21022`)
 -
 -
 
diff --git a/pandas/core/arrays/categorical.py b/pandas/core/arrays/categorical.py
index 30f9c56d24f02..3791f854ea7c2 100644
--- a/pandas/core/arrays/categorical.py
+++ b/pandas/core/arrays/categorical.py
@@ -1847,6 +1847,19 @@ def __iter__(self):
         """Returns an Iterator over the values of this Categorical."""
         return iter(self.get_values().tolist())
 
+    def __contains__(self, key):
+        """Returns True if `key` is in this Categorical."""
+        hash(key)
+        if isna(key):
+            return self.isna().any()
+        elif self.categories._defer_to_indexing:  # e.g. Interval values
+            loc = self.categories.get_loc(key)
+            return np.isin(self.codes, loc).any()
+        elif key in self.categories:
+            return self.categories.get_loc(key) in self._codes
+        else:
+            return False
+
     def _tidy_repr(self, max_vals=10, footer=True):
         """ a short repr displaying only max_vals and an optional (but default
         footer)
diff --git a/pandas/core/indexes/category.py b/pandas/core/indexes/category.py
index 150eca32e229d..8722170ac41d4 100644
--- a/pandas/core/indexes/category.py
+++ b/pandas/core/indexes/category.py
@@ -323,20 +323,10 @@ def _reverse_indexer(self):
 
     @Appender(_index_shared_docs['__contains__'] % _index_doc_kwargs)
     def __contains__(self, key):
-        hash(key)
-
-        if self.categories._defer_to_indexing:
-            return key in self.categories
-
         return key in self.values
 
     @Appender(_index_shared_docs['contains'] % _index_doc_kwargs)
     def contains(self, key):
-        hash(key)
-
-        if self.categories._defer_to_indexing:
-            return self.categories.contains(key)
-
         return key in self.values
 
     def __array__(self, dtype=None):
diff --git a/pandas/tests/indexes/test_category.py b/pandas/tests/indexes/test_category.py
index a2a4170256088..88d76210da8ba 100644
--- a/pandas/tests/indexes/test_category.py
+++ b/pandas/tests/indexes/test_category.py
@@ -244,6 +244,17 @@ def test_contains(self):
             list('aabbca') + [np.nan], categories=list('cabdef'))
         assert np.nan in ci
 
+        ci = CategoricalIndex(
+            list('aaa'), categories=list('cabdef'))
+        assert 'f' not in ci
+
+    def test_containst_defer_to_indexing(self):
+        intervals = pd.interval_range(1, 4)
+        cat = pd.CategoricalIndex(list(intervals[:-1]), categories=intervals)
+        assert intervals[0] in cat
+        assert intervals[1] in cat
+        assert intervals[2] not in cat
+
     def test_min_max(self):
 
         ci = self.create_index(ordered=False)