diff --git a/changelog.d/13514.bugfix b/changelog.d/13514.bugfix
new file mode 100644
index 000000000000..7498af0e472a
--- /dev/null
+++ b/changelog.d/13514.bugfix
@@ -0,0 +1 @@
+Faster room joins: make `/joined_members` block whilst the room is partial stated.
\ No newline at end of file
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 6b0360359850..8f29ee9a8737 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -331,7 +331,11 @@ async def get_joined_members(self, requester: Requester, room_id: str) -> dict:
                     msg="Getting joined members while not being a current member of the room is forbidden.",
                 )
 
-        users_with_profile = await self.store.get_users_in_room_with_profiles(room_id)
+        users_with_profile = (
+            await self._state_storage_controller.get_users_in_room_with_profiles(
+                room_id
+            )
+        )
 
         # If this is an AS, double check that they are allowed to see the members.
         # This can either be because the AS user is in the room or because there
diff --git a/synapse/storage/controllers/state.py b/synapse/storage/controllers/state.py
index 0d480f101432..0c78eb735e42 100644
--- a/synapse/storage/controllers/state.py
+++ b/synapse/storage/controllers/state.py
@@ -30,6 +30,7 @@
 from synapse.api.constants import EventTypes
 from synapse.events import EventBase
 from synapse.logging.opentracing import trace
+from synapse.storage.roommember import ProfileInfo
 from synapse.storage.state import StateFilter
 from synapse.storage.util.partial_state_events_tracker import (
     PartialCurrentStateTracker,
@@ -506,3 +507,15 @@ async def get_current_hosts_in_room(self, room_id: str) -> Set[str]:
         await self._partial_state_room_tracker.await_full_state(room_id)
 
         return await self.stores.main.get_current_hosts_in_room(room_id)
+
+    async def get_users_in_room_with_profiles(
+        self, room_id: str
+    ) -> Dict[str, ProfileInfo]:
+        """
+        Get the current users in the room with their profiles.
+        If the room is currently partial-stated, this will block until the room has
+        full state.
+        """
+        await self._partial_state_room_tracker.await_full_state(room_id)
+
+        return await self.stores.main.get_users_in_room_with_profiles(room_id)
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index 93ff4816c804..5e5f607a14c8 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -283,6 +283,9 @@ async def get_users_in_room_with_profiles(
 
         Returns:
             A mapping from user ID to ProfileInfo.
+
+        Preconditions:
+          - There is full state available for the room (it is not partial-stated).
         """
 
         def _get_users_in_room_with_profiles(