diff --git a/synapse/storage/devices.py b/synapse/storage/devices.py
index 7b5903bf8..81c43d31f 100644
--- a/synapse/storage/devices.py
+++ b/synapse/storage/devices.py
@@ -33,6 +33,13 @@ class DeviceStore(SQLBaseStore):
             self._prune_old_outbound_device_pokes, 60 * 60 * 1000
         )
 
+        self.register_background_index_update(
+            "device_lists_stream_idx",
+            index_name="device_lists_stream_user_id",
+            table="device_lists_stream",
+            columns=["user_id", "device_id"],
+        )
+
     @defer.inlineCallbacks
     def store_device(self, user_id, device_id,
                      initial_device_display_name):
@@ -546,6 +553,16 @@ class DeviceStore(SQLBaseStore):
                 host, stream_id,
             )
 
+        # Delete older entries in the table, as we really only care about
+        # when the latest change happened.
+        txn.executemany(
+            """
+            DELETE FROM device_lists_stream
+            WHERE user_id = ? AND device_id = ? AND stream_id < ?
+            """,
+            [(user_id, device_id, stream_id) for device_id in device_ids]
+        )
+
         self._simple_insert_many_txn(
             txn,
             table="device_lists_stream",
diff --git a/synapse/storage/schema/delta/41/device_list_stream_idx.sql b/synapse/storage/schema/delta/41/device_list_stream_idx.sql
new file mode 100644
index 000000000..b7bee8b69
--- /dev/null
+++ b/synapse/storage/schema/delta/41/device_list_stream_idx.sql
@@ -0,0 +1,17 @@
+/* Copyright 2017 Vector Creations Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+INSERT into background_updates (update_name, progress_json)
+    VALUES ('device_lists_stream_idx', '{}');