diff --git a/benchmark/benchmark.py b/benchmark/benchmark.py
index fbf8bc62a..67eaa0d3e 100644
--- a/benchmark/benchmark.py
+++ b/benchmark/benchmark.py
@@ -1,4 +1,4 @@
-from typing import Any, Dict
+from typing import Any, Dict, List
 
 from pytest import lazy_fixture  # type: ignore
 from pytest import fixture, mark, param
@@ -6,7 +6,7 @@
 from omegaconf import OmegaConf
 
 
-def build(
+def build_dict(
     d: Dict[str, Any], depth: int, width: int, leaf_value: Any = 1
 ) -> Dict[str, Any]:
     if depth == 0:
@@ -16,24 +16,28 @@ def build(
         for i in range(width):
             c: Dict[str, Any] = {}
             d[f"key_{i}"] = c
-            build(c, depth - 1, width, leaf_value)
+            build_dict(c, depth - 1, width, leaf_value)
 
     return d
 
 
+def build_list(length: int, val: Any = 1) -> List[int]:
+    return [val] * length
+
+
 @fixture(scope="module")
 def large_dict() -> Any:
-    return build({}, 11, 2)
+    return build_dict({}, 11, 2)
 
 
 @fixture(scope="module")
 def small_dict() -> Any:
-    return build({}, 5, 2)
+    return build_dict({}, 5, 2)
 
 
 @fixture(scope="module")
 def dict_with_list_leaf() -> Any:
-    return build({}, 5, 2, leaf_value=[1, 2])
+    return build_dict({}, 5, 2, leaf_value=[1, 2])
 
 
 @fixture(scope="module")
@@ -56,6 +60,16 @@ def merge_data(small_dict: Any) -> Any:
     return [OmegaConf.create(small_dict) for _ in range(5)]
 
 
+@fixture(scope="module")
+def small_list() -> Any:
+    return build_list(3, 1)
+
+
+@fixture(scope="module")
+def small_listconfig(small_list: Any) -> Any:
+    return OmegaConf.create(small_list)
+
+
 @mark.parametrize(
     "data",
     [
@@ -79,3 +93,26 @@ def test_omegaconf_create(data: Any, benchmark: Any) -> None:
 )
 def test_omegaconf_merge(merge_function: Any, merge_data: Any, benchmark: Any) -> None:
     benchmark(merge_function, merge_data)
+
+
+@mark.parametrize(
+    "lst",
+    [
+        lazy_fixture("small_list"),
+        lazy_fixture("small_listconfig"),
+    ],
+)
+def test_list_in(lst: List[Any], benchmark: Any) -> None:
+    benchmark(lambda seq, val: val in seq, lst, 10)
+
+
+@mark.parametrize(
+    "lst",
+    [
+        lazy_fixture("small_list"),
+        lazy_fixture("small_listconfig"),
+    ],
+)
+def test_list_iter(lst: List[Any], benchmark: Any) -> None:
+    # Performance is really bad for list config iteration, not sure why.
+    benchmark(lambda seq: [x for x in seq], lst)
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 e8cd8c683..ef7bddb84 100644
--- a/tests/test_basic_ops_list.py
+++ b/tests/test_basic_ops_list.py
@@ -114,6 +114,37 @@ def test_in_list() -> None:
     assert "blah" not in c
 
 
+def test_in_with_interpolation() -> None:
+    c = OmegaConf.create({"a": ["${b}"], "b": 10})
+    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)