diff --git a/news/529.feature b/news/529.feature
new file mode 100644
index 000000000..e5b05dbb4
--- /dev/null
+++ b/news/529.feature
@@ -0,0 +1 @@
+ListConfig.__contains__ optimized, about 15x faster in a benchmark
diff --git a/omegaconf/listconfig.py b/omegaconf/listconfig.py
index 1b6d2cc49..5c1aaee90 100644
--- a/omegaconf/listconfig.py
+++ b/omegaconf/listconfig.py
@@ -539,7 +539,18 @@ def __iadd__(self, other: Iterable[Any]) -> "ListConfig":
         return self
 
     def __contains__(self, item: Any) -> bool:
-        for x in iter(self):
+        if self._is_none():
+            raise TypeError(
+                "Cannot check if an item is in a ListConfig object representing None"
+            )
+        if self._is_missing():
+            raise MissingMandatoryValue(
+                "Cannot check if an item is in missing ListConfig"
+            )
+
+        lst = self.__dict__["_content"]
+        for x in lst:
+            x = x._dereference_node()
             if x == item:
                 return True
         return False
diff --git a/tests/test_basic_ops_list.py b/tests/test_basic_ops_list.py
index 2bed50cc1..ef7bddb84 100644
--- a/tests/test_basic_ops_list.py
+++ b/tests/test_basic_ops_list.py
@@ -119,6 +119,32 @@ def test_in_with_interpolation() -> None:
     assert 10 in c.a
 
 
+@pytest.mark.parametrize(
+    ("lst", "expected"),
+    [
+        pytest.param(
+            ListConfig(content=None),
+            pytest.raises(
+                TypeError,
+                match="Cannot check if an item is in a ListConfig object representing None",
+            ),
+            id="ListConfig(None)",
+        ),
+        pytest.param(
+            ListConfig(content="???"),
+            pytest.raises(
+                MissingMandatoryValue,
+                match="Cannot check if an item is in missing ListConfig",
+            ),
+            id="ListConfig(???)",
+        ),
+    ],
+)
+def test_not_in_special_lists(lst: Any, expected: Any) -> None:
+    with expected:
+        "foo" not in lst
+
+
 def test_list_config_with_list() -> None:
     c = OmegaConf.create([])
     assert isinstance(c, ListConfig)