diff --git a/changelog.d/14606.misc b/changelog.d/14606.misc
new file mode 100644
index 000000000000..c6930c5721d4
--- /dev/null
+++ b/changelog.d/14606.misc
@@ -0,0 +1 @@
+Faster joins: handle join during fast join.
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index eca75f1108d1..0d2e1700b949 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -599,6 +599,8 @@ async def do_invite_join(
             except ValueError:
                 pass
 
+            already_partially_joined = await self.store.is_partial_state_room(room_id)
+
             ret = await self.federation_client.send_join(
                 host_list, event, room_version_obj
             )
@@ -629,7 +631,7 @@ async def do_invite_join(
                 state_events=state,
             )
 
-            if ret.partial_state:
+            if ret.partial_state and not already_partially_joined:
                 # Mark the room as having partial state.
                 # The background process is responsible for unmarking this flag,
                 # even if the join fails.
@@ -676,7 +678,7 @@ async def do_invite_join(
                 # state for the room.
                 # If the join failed, the background process is responsible for
                 # cleaning up — including unmarking the room as a partial state room.
-                if ret.partial_state:
+                if ret.partial_state and not already_partially_joined:
                     # Kick off the process of asynchronously fetching the state for this
                     # room.
                     run_as_background_process(
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index d236cc09b526..6978e49a1023 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -821,10 +821,25 @@ async def update_membership_locked(
                 origin_server_ts=origin_server_ts,
             )
 
+        is_partial_state_room = await self.store.is_partial_state_room(room_id)
+
+        await_full_state = True
+        # Join is ok because instead of relying on the events to validate it if the
+        # server is already partially joined, we will do make_join-send_join
+        #
+        # Leave doesn't really need validation: if the target has been banned in the
+        # meantime, we should have received the event and if not, state res at fed
+        # level will take care of solving the discrepancy.
+        if is_partial_state_room:
+            if get_domain_from_id(
+                target.to_string()
+            ) == self._server_name and action in [Membership.JOIN, Membership.LEAVE]:
+                await_full_state = False
+
         latest_event_ids = await self.store.get_prev_events_for_room(room_id)
 
         state_before_join = await self.state_handler.compute_state_after_events(
-            room_id, latest_event_ids
+            room_id, latest_event_ids, await_full_state=await_full_state
         )
 
         # TODO: Refactor into dictionary of explicitly allowed transitions