diff --git a/changelog.d/8296.misc b/changelog.d/8296.misc
new file mode 100644
index 0000000000..f593a5b347
--- /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 69650ff221..7294649d71 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 90189869cc..4e658d9a48 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -1235,7 +1235,7 @@ class AuthHandler(BaseHandler):
         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 d629c7c16c..dd40fd1299 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 a5734bebab..262901363f 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -86,7 +86,7 @@ from synapse.visibility import filter_events_for_server
 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 8715abd4d1..285c481a96 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 a615c7c2f0..9b3a4f638b 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 @@ class JoinedSyncResult:
             # 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 @@ class SyncResult:
             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 e6f067ca29..a306faa267 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 5eaf3151ce..3c86cbc546 100644
--- a/synapse/http/matrixfederationclient.py
+++ b/synapse/http/matrixfederationclient.py
@@ -76,7 +76,7 @@ MAXINT = sys.maxsize
 _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 22598e02d2..2e282d9d67 100644
--- a/synapse/logging/context.py
+++ b/synapse/logging/context.py
@@ -217,11 +217,9 @@ class _Sentinel:
     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 2643380d9e..a1f7ca3449 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -59,7 +59,7 @@ class RegistryProxy:
                 yield metric
 
 
-@attr.s(hash=True)
+@attr.s(slots=True, hash=True)
 class LaterGauge:
 
     name = attr.ib(type=str)
@@ -205,7 +205,7 @@ class InFlightGauge:
         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 12cd84b27b..a8fd3ef886 100644
--- a/synapse/notifier.py
+++ b/synapse/notifier.py
@@ -164,11 +164,9 @@ class _NotifierUserStream:
 
 
 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 682d47f402..1f609f158c 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 cd8c246594..987765e877 100644
--- a/synapse/rest/media/v1/preview_url_resource.py
+++ b/synapse/rest/media/v1/preview_url_resource.py
@@ -102,7 +102,7 @@ for endpoint, globs in _oembed_globs.items():
         _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 c7e3015b5d..56d6afb863 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 fba3098ea2..c8df0bcb3f 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 @@ if TYPE_CHECKING:
     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 5233ed83e2..7805fb814e 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 b89668d561..3b9211a6d2 100644
--- a/synapse/storage/databases/main/ui_auth.py
+++ b/synapse/storage/databases/main/ui_auth.py
@@ -23,7 +23,7 @@ from synapse.types import JsonDict
 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 a7f2dfb850..4957e77f4c 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 d30e3f11e7..cec96ad6a7 100644
--- a/synapse/storage/relations.py
+++ b/synapse/storage/relations.py
@@ -22,7 +22,7 @@ from synapse.api.errors import SynapseError
 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 60ecc498ab..d55b93d763 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 237f588658..8fc05be278 100644
--- a/synapse/util/caches/__init__.py
+++ b/synapse/util/caches/__init__.py
@@ -42,7 +42,7 @@ response_cache_evicted = Gauge(
 response_cache_total = Gauge("synapse_util_caches_response_cache:total", "", ["name"])
 
 
-@attr.s
+@attr.s(slots=True)
 class CacheMetric:
 
     _cache = attr.ib()