diff --git a/rock_paper_sand/justwatch.py b/rock_paper_sand/justwatch.py
index 04c1a5d..374315b 100644
--- a/rock_paper_sand/justwatch.py
+++ b/rock_paper_sand/justwatch.py
@@ -197,7 +197,7 @@ def _content_number(content: Any) -> multi_level_set.MultiLevelNumber:
     return multi_level_set.MultiLevelNumber(tuple(parts))
 
 
-class Filter(media_filter.Filter):
+class Filter(media_filter.CachedFilter):
     """Filter based on JustWatch's API."""
 
     def __init__(
@@ -206,6 +206,7 @@ def __init__(
         *,
         api: Api,
     ) -> None:
+        super().__init__()
         self._config = filter_config
         self._api = api
 
@@ -292,7 +293,9 @@ def _all_done(
             return False
         return True
 
-    def filter(self, item: media_item.MediaItem) -> media_filter.FilterResult:
+    def filter_implementation(
+        self, item: media_item.MediaItem
+    ) -> media_filter.FilterResult:
         """See base class."""
         now = datetime.datetime.now(tz=datetime.timezone.utc)
         if not item.proto.justwatch_id:
diff --git a/rock_paper_sand/media_filter.py b/rock_paper_sand/media_filter.py
index 14e5190..4cc5879 100644
--- a/rock_paper_sand/media_filter.py
+++ b/rock_paper_sand/media_filter.py
@@ -48,6 +48,27 @@ def filter(self, item: media_item.MediaItem) -> FilterResult:
         raise NotImplementedError()
 
 
+class CachedFilter(Filter, abc.ABC):
+    """Base class for filters that cache their results.
+
+    Child classes should override filter_implementation() instead of filter().
+    """
+
+    def __init__(self) -> None:
+        self._result_by_id: dict[str, FilterResult] = {}
+
+    @abc.abstractmethod
+    def filter_implementation(self, item: media_item.MediaItem) -> FilterResult:
+        """See Filter.filter."""
+        raise NotImplementedError()
+
+    def filter(self, item: media_item.MediaItem) -> FilterResult:
+        """See base class."""
+        if item.id not in self._result_by_id:
+            self._result_by_id[item.id] = self.filter_implementation(item)
+        return self._result_by_id[item.id]
+
+
 class Not(Filter):
     """Inverts another filter."""
 
diff --git a/rock_paper_sand/media_filter_test.py b/rock_paper_sand/media_filter_test.py
index 680def8..7f88019 100644
--- a/rock_paper_sand/media_filter_test.py
+++ b/rock_paper_sand/media_filter_test.py
@@ -28,12 +28,23 @@
 from rock_paper_sand.proto import config_pb2
 
 
-class _ExtraInfoFilter(media_filter.Filter):
+class _ExtraInfoFilter(media_filter.CachedFilter):
+    """Test filter that returns the given extra info.
+
+    Attributes:
+        call_count: Number of times filter_implementation() was called.
+    """
+
     def __init__(self, extra: Set[str]) -> None:
+        super().__init__()
         self._extra = extra
+        self.call_count = 0
 
-    def filter(self, item: media_item.MediaItem) -> media_filter.FilterResult:
+    def filter_implementation(
+        self, item: media_item.MediaItem
+    ) -> media_filter.FilterResult:
         """See base class."""
+        self.call_count += 1
         return media_filter.FilterResult(True, extra=self._extra)
 
 
@@ -231,6 +242,18 @@ def test_basic_filter(
         result = test_filter.filter(media_item.MediaItem.from_config(item))
         self.assertEqual(expected_result, result)
 
+    def test_cached_filter(self) -> None:
+        test_filter = _ExtraInfoFilter({"foo"})
+        item = media_item.MediaItem.from_config(config_pb2.MediaItem())
+        expected_result = media_filter.FilterResult(True, extra={"foo"})
+
+        first_result = test_filter.filter(item)
+        second_result = test_filter.filter(item)
+
+        self.assertEqual(expected_result, first_result)
+        self.assertEqual(expected_result, second_result)
+        self.assertEqual(1, test_filter.call_count)
+
     def test_justwatch_filter(self) -> None:
         mock_filter = mock.create_autospec(
             media_filter.Filter, spec_set=True, instance=True
diff --git a/rock_paper_sand/media_item.py b/rock_paper_sand/media_item.py
index 74cde02..7a4a2c8 100644
--- a/rock_paper_sand/media_item.py
+++ b/rock_paper_sand/media_item.py
@@ -16,6 +16,7 @@
 from collections.abc import Sequence
 import dataclasses
 from typing import Self
+import uuid
 
 from rock_paper_sand import multi_level_set
 from rock_paper_sand.proto import config_pb2
@@ -26,11 +27,18 @@ class MediaItem:
     """Media item.
 
     Attributes:
+        id: Unique ID of the media item. This is not stable across runs of the
+            program, so it should not be stored anywhere or shown to the user.
+            It's designed for caching filter results in memory.
         proto: Proto from the config file.
         done: Parsed proto.done field.
         parts: Parsed proto.parts field.
     """
 
+    id: str = dataclasses.field(
+        default_factory=lambda: str(uuid.uuid4()),
+        repr=False,
+    )
     proto: config_pb2.MediaItem
     done: multi_level_set.MultiLevelSet
     parts: Sequence["MediaItem"]
diff --git a/rock_paper_sand/media_item_test.py b/rock_paper_sand/media_item_test.py
index 61bad39..e76b478 100644
--- a/rock_paper_sand/media_item_test.py
+++ b/rock_paper_sand/media_item_test.py
@@ -42,10 +42,12 @@ def test_from_config(self) -> None:
 
         self.assertEqual(
             media_item.MediaItem(
+                id=mock.ANY,
                 proto=proto,
                 done=mock.ANY,
                 parts=(
                     media_item.MediaItem(
+                        id=mock.ANY,
                         proto=config_pb2.MediaItem(name="some-part"),
                         done=mock.ANY,
                         parts=(),
@@ -57,6 +59,12 @@ def test_from_config(self) -> None:
         self.assertIn(multi_level_set.parse_number("1"), item.done)
         self.assertNotIn(multi_level_set.parse_number("1"), item.parts[0].done)
 
+    def test_id(self) -> None:
+        self.assertNotEqual(
+            media_item.MediaItem.from_config(config_pb2.MediaItem()),
+            media_item.MediaItem.from_config(config_pb2.MediaItem()),
+        )
+
 
 if __name__ == "__main__":
     absltest.main()