diff --git a/changelog.d/8296.misc b/changelog.d/8296.misc
new file mode 100644
index 000000000000..f593a5b34778
--- /dev/null
+++ b/changelog.d/8296.misc
@@ -0,0 +1 @@
+Use slotted classes where possible.
diff --git a/synapse/handlers/acme_issuing_service.py b/synapse/handlers/acme_issuing_service.py
index 69650ff221a9..7294649d717a 100644
--- a/synapse/handlers/acme_issuing_service.py
+++ b/synapse/handlers/acme_issuing_service.py
@@ -76,7 +76,7 @@ def create_issuing_service(reactor, acme_url, account_key_file, well_known_resou
     )
 
 
-@attr.s
+@attr.s(slots=True)
 @implementer(ICertificateStore)
 class ErsatzStore:
     """
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 90189869cc4a..4e658d9a4879 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -1235,7 +1235,7 @@ def add_query_param_to_url(url: str, param_name: str, param: Any):
         return urllib.parse.urlunparse(url_parts)
 
 
-@attr.s
+@attr.s(slots=True)
 class MacaroonGenerator:
 
     hs = attr.ib()
diff --git a/synapse/handlers/e2e_keys.py b/synapse/handlers/e2e_keys.py
index d629c7c16c0d..dd40fd129936 100644
--- a/synapse/handlers/e2e_keys.py
+++ b/synapse/handlers/e2e_keys.py
@@ -1201,7 +1201,7 @@ def _one_time_keys_match(old_key_json, new_key):
     return old_key == new_key_copy
 
 
-@attr.s
+@attr.s(slots=True)
 class SignatureListItem:
     """An item in the signature list as used by upload_signatures_for_device_keys.
     """
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index a5734bebab3b..262901363f63 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -86,7 +86,7 @@
 logger = logging.getLogger(__name__)
 
 
-@attr.s
+@attr.s(slots=True)
 class _NewEventInfo:
     """Holds information about a received event, ready for passing to _handle_new_events
 
diff --git a/synapse/handlers/saml_handler.py b/synapse/handlers/saml_handler.py
index 8715abd4d1c7..285c481a9604 100644
--- a/synapse/handlers/saml_handler.py
+++ b/synapse/handlers/saml_handler.py
@@ -46,7 +46,7 @@ class MappingException(Exception):
     """Used to catch errors when mapping the SAML2 response to a user."""
 
 
-@attr.s
+@attr.s(slots=True)
 class Saml2SessionData:
     """Data we track about SAML2 sessions"""
 
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index a615c7c2f0f1..9b3a4f638b13 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -89,14 +89,12 @@ class TimelineBatch:
     events = attr.ib(type=List[EventBase])
     limited = attr.ib(bool)
 
-    def __nonzero__(self) -> bool:
+    def __bool__(self) -> bool:
         """Make the result appear empty if there are no updates. This is used
         to tell if room needs to be part of the sync result.
         """
         return bool(self.events)
 
-    __bool__ = __nonzero__  # python3
-
 
 # We can't freeze this class, because we need to update it after it's instantiated to
 # update its unread count. This is because we calculate the unread count for a room only
@@ -114,7 +112,7 @@ class JoinedSyncResult:
     summary = attr.ib(type=Optional[JsonDict])
     unread_count = attr.ib(type=int)
 
-    def __nonzero__(self) -> bool:
+    def __bool__(self) -> bool:
         """Make the result appear empty if there are no updates. This is used
         to tell if room needs to be part of the sync result.
         """
@@ -127,8 +125,6 @@ def __nonzero__(self) -> bool:
             # else in the result, we don't need to send it.
         )
 
-    __bool__ = __nonzero__  # python3
-
 
 @attr.s(slots=True, frozen=True)
 class ArchivedSyncResult:
@@ -137,26 +133,22 @@ class ArchivedSyncResult:
     state = attr.ib(type=StateMap[EventBase])
     account_data = attr.ib(type=List[JsonDict])
 
-    def __nonzero__(self) -> bool:
+    def __bool__(self) -> bool:
         """Make the result appear empty if there are no updates. This is used
         to tell if room needs to be part of the sync result.
         """
         return bool(self.timeline or self.state or self.account_data)
 
-    __bool__ = __nonzero__  # python3
-
 
 @attr.s(slots=True, frozen=True)
 class InvitedSyncResult:
     room_id = attr.ib(type=str)
     invite = attr.ib(type=EventBase)
 
-    def __nonzero__(self) -> bool:
+    def __bool__(self) -> bool:
         """Invited rooms should always be reported to the client"""
         return True
 
-    __bool__ = __nonzero__  # python3
-
 
 @attr.s(slots=True, frozen=True)
 class GroupsSyncResult:
@@ -164,11 +156,9 @@ class GroupsSyncResult:
     invite = attr.ib(type=JsonDict)
     leave = attr.ib(type=JsonDict)
 
-    def __nonzero__(self) -> bool:
+    def __bool__(self) -> bool:
         return bool(self.join or self.invite or self.leave)
 
-    __bool__ = __nonzero__  # python3
-
 
 @attr.s(slots=True, frozen=True)
 class DeviceLists:
@@ -181,13 +171,11 @@ class DeviceLists:
     changed = attr.ib(type=Collection[str])
     left = attr.ib(type=Collection[str])
 
-    def __nonzero__(self) -> bool:
+    def __bool__(self) -> bool:
         return bool(self.changed or self.left)
 
-    __bool__ = __nonzero__  # python3
 
-
-@attr.s
+@attr.s(slots=True)
 class _RoomChanges:
     """The set of room entries to include in the sync, plus the set of joined
     and left room IDs since last sync.
@@ -227,7 +215,7 @@ class SyncResult:
     device_one_time_keys_count = attr.ib(type=JsonDict)
     groups = attr.ib(type=Optional[GroupsSyncResult])
 
-    def __nonzero__(self) -> bool:
+    def __bool__(self) -> bool:
         """Make the result appear empty if there are no updates. This is used
         to tell if the notifier needs to wait for more events when polling for
         events.
@@ -243,8 +231,6 @@ def __nonzero__(self) -> bool:
             or self.groups
         )
 
-    __bool__ = __nonzero__  # python3
-
 
 class SyncHandler:
     def __init__(self, hs: "HomeServer"):
@@ -2038,7 +2024,7 @@ def _calculate_state(
     return {event_id_to_key[e]: e for e in state_ids}
 
 
-@attr.s
+@attr.s(slots=True)
 class SyncResultBuilder:
     """Used to help build up a new SyncResult for a user
 
@@ -2074,7 +2060,7 @@ class SyncResultBuilder:
     to_device = attr.ib(type=List[JsonDict], default=attr.Factory(list))
 
 
-@attr.s
+@attr.s(slots=True)
 class RoomSyncResultBuilder:
     """Stores information needed to create either a `JoinedSyncResult` or
     `ArchivedSyncResult`.
diff --git a/synapse/http/federation/well_known_resolver.py b/synapse/http/federation/well_known_resolver.py
index e6f067ca29f0..a306faa267c1 100644
--- a/synapse/http/federation/well_known_resolver.py
+++ b/synapse/http/federation/well_known_resolver.py
@@ -311,7 +311,7 @@ def _parse_cache_control(headers: Headers) -> Dict[bytes, Optional[bytes]]:
     return cache_controls
 
 
-@attr.s()
+@attr.s(slots=True)
 class _FetchWellKnownFailure(Exception):
     # True if we didn't get a non-5xx HTTP response, i.e. this may or may not be
     # a temporary failure.
diff --git a/synapse/http/matrixfederationclient.py b/synapse/http/matrixfederationclient.py
index 5eaf3151ce24..3c86cbc546db 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -76,7 +76,7 @@
 _next_id = 1
 
 
-@attr.s(frozen=True)
+@attr.s(slots=True, frozen=True)
 class MatrixFederationRequest:
     method = attr.ib()
     """HTTP method
diff --git a/synapse/logging/context.py b/synapse/logging/context.py
index 22598e02d22a..2e282d9d670e 100644
--- a/synapse/logging/context.py
+++ b/synapse/logging/context.py
@@ -217,11 +217,9 @@ def add_database_scheduled(self, sched_sec):
     def record_event_fetch(self, event_count):
         pass
 
-    def __nonzero__(self):
+    def __bool__(self):
         return False
 
-    __bool__ = __nonzero__  # python3
-
 
 SENTINEL_CONTEXT = _Sentinel()
 
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 2643380d9e96..a1f7ca344924 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -59,7 +59,7 @@ def collect():
                 yield metric
 
 
-@attr.s(hash=True)
+@attr.s(slots=True, hash=True)
 class LaterGauge:
 
     name = attr.ib(type=str)
@@ -205,7 +205,7 @@ def _register_with_collector(self):
         all_gauges[self.name] = self
 
 
-@attr.s(hash=True)
+@attr.s(slots=True, hash=True)
 class BucketCollector:
     """
     Like a Histogram, but allows buckets to be point-in-time instead of
diff --git a/synapse/notifier.py b/synapse/notifier.py
index 12cd84b27bfe..a8fd3ef886ce 100644
--- a/synapse/notifier.py
+++ b/synapse/notifier.py
@@ -164,11 +164,9 @@ def new_listener(self, token: StreamToken) -> _NotificationListener:
 
 
 class EventStreamResult(namedtuple("EventStreamResult", ("events", "tokens"))):
-    def __nonzero__(self):
+    def __bool__(self):
         return bool(self.events)
 
-    __bool__ = __nonzero__  # python3
-
 
 class Notifier:
     """ This class is responsible for notifying any listeners when there are
diff --git a/synapse/replication/tcp/streams/_base.py b/synapse/replication/tcp/streams/_base.py
index 682d47f402be..1f609f158c14 100644
--- a/synapse/replication/tcp/streams/_base.py
+++ b/synapse/replication/tcp/streams/_base.py
@@ -383,7 +383,7 @@ class CachesStream(Stream):
     the cache on the workers
     """
 
-    @attr.s
+    @attr.s(slots=True)
     class CachesStreamRow:
         """Stream to inform workers they should invalidate their cache.
 
@@ -441,7 +441,7 @@ class DeviceListsStream(Stream):
     told about a device update.
     """
 
-    @attr.s
+    @attr.s(slots=True)
     class DeviceListsStreamRow:
         entity = attr.ib(type=str)
 
diff --git a/synapse/rest/media/v1/preview_url_resource.py b/synapse/rest/media/v1/preview_url_resource.py
index cd8c246594cf..987765e8770f 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -102,7 +102,7 @@
         _oembed_patterns[re.compile(pattern)] = endpoint
 
 
-@attr.s
+@attr.s(slots=True)
 class OEmbedResult:
     # Either HTML content or URL must be provided.
     html = attr.ib(type=Optional[str])
diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py
index c7e3015b5dbe..56d6afb86353 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -678,7 +678,7 @@ def resolve_events_with_store(
         )
 
 
-@attr.s
+@attr.s(slots=True)
 class StateResolutionStore:
     """Interface that allows state resolution algorithms to access the database
     in well defined way.
diff --git a/synapse/storage/databases/main/end_to_end_keys.py b/synapse/storage/databases/main/end_to_end_keys.py
index fba3098ea2ab..c8df0bcb3fe5 100644
--- a/synapse/storage/databases/main/end_to_end_keys.py
+++ b/synapse/storage/databases/main/end_to_end_keys.py
@@ -35,7 +35,7 @@
     from synapse.handlers.e2e_keys import SignatureListItem
 
 
-@attr.s
+@attr.s(slots=True)
 class DeviceKeyLookupResult:
     """The type returned by get_e2e_device_keys_and_signatures"""
 
diff --git a/synapse/storage/databases/main/event_push_actions.py b/synapse/storage/databases/main/event_push_actions.py
index 5233ed83e29b..7805fb814e30 100644
--- a/synapse/storage/databases/main/event_push_actions.py
+++ b/synapse/storage/databases/main/event_push_actions.py
@@ -969,7 +969,7 @@ def _action_has_highlight(actions):
     return False
 
 
-@attr.s
+@attr.s(slots=True)
 class _EventPushSummary:
     """Summary of pending event push actions for a given user in a given room.
     Used in _rotate_notifs_before_txn to manipulate results from event_push_actions.
diff --git a/synapse/storage/databases/main/ui_auth.py b/synapse/storage/databases/main/ui_auth.py
index b89668d561b1..3b9211a6d235 100644
--- a/synapse/storage/databases/main/ui_auth.py
+++ b/synapse/storage/databases/main/ui_auth.py
@@ -23,7 +23,7 @@
 from synapse.util import json_encoder, stringutils
 
 
-@attr.s
+@attr.s(slots=True)
 class UIAuthSessionData:
     session_id = attr.ib(type=str)
     # The dictionary from the client root level, not the 'auth' key.
diff --git a/synapse/storage/prepare_database.py b/synapse/storage/prepare_database.py
index a7f2dfb85061..4957e77f4c27 100644
--- a/synapse/storage/prepare_database.py
+++ b/synapse/storage/prepare_database.py
@@ -638,7 +638,7 @@ def _get_or_create_schema_state(txn, database_engine):
     return None
 
 
-@attr.s()
+@attr.s(slots=True)
 class _DirectoryListing:
     """Helper class to store schema file name and the
     absolute path to it.
diff --git a/synapse/storage/relations.py b/synapse/storage/relations.py
index d30e3f11e7aa..cec96ad6a72e 100644
--- a/synapse/storage/relations.py
+++ b/synapse/storage/relations.py
@@ -22,7 +22,7 @@
 logger = logging.getLogger(__name__)
 
 
-@attr.s
+@attr.s(slots=True)
 class PaginationChunk:
     """Returned by relation pagination APIs.
 
diff --git a/synapse/util/__init__.py b/synapse/util/__init__.py
index 60ecc498ab78..d55b93d76385 100644
--- a/synapse/util/__init__.py
+++ b/synapse/util/__init__.py
@@ -45,7 +45,7 @@ def unwrapFirstError(failure):
     return failure.value.subFailure
 
 
-@attr.s
+@attr.s(slots=True)
 class Clock:
     """
     A Clock wraps a Twisted reactor and provides utilities on top of it.
diff --git a/synapse/util/caches/__init__.py b/synapse/util/caches/__init__.py
index 237f5886582b..8fc05be278fa 100644
--- a/synapse/util/caches/__init__.py
+++ b/synapse/util/caches/__init__.py
@@ -42,7 +42,7 @@
 response_cache_total = Gauge("synapse_util_caches_response_cache:total", "", ["name"])
 
 
-@attr.s
+@attr.s(slots=True)
 class CacheMetric:
 
     _cache = attr.ib()