diff --git a/changelog.d/6141.bugfix b/changelog.d/6141.bugfix
new file mode 100644
index 0000000000..c93920b7b5
--- /dev/null
+++ b/changelog.d/6141.bugfix
@@ -0,0 +1 @@
+Fix bad performance of censoring redactions background task.
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index ddf7ab6479..2e485c8644 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -1389,6 +1389,18 @@ class EventsStore(
             ],
         )
 
+        for event, _ in events_and_contexts:
+            if not event.internal_metadata.is_redacted():
+                # If we're persisting an unredacted event we go and ensure
+                # that we mark any redactions that reference this event as
+                # requiring censoring.
+                self._simple_update_txn(
+                    txn,
+                    table="redactions",
+                    keyvalues={"redacts": event.event_id},
+                    updatevalues={"have_censored": False},
+                )
+
     def _store_rejected_events_txn(self, txn, events_and_contexts):
         """Add rows to the 'rejections' table for received events which were
         rejected
@@ -1552,9 +1564,15 @@ class EventsStore(
     def _store_redaction(self, txn, event):
         # invalidate the cache for the redacted event
         txn.call_after(self._invalidate_get_event_cache, event.redacts)
-        txn.execute(
-            "INSERT INTO redactions (event_id, redacts) VALUES (?,?)",
-            (event.event_id, event.redacts),
+
+        self._simple_insert_txn(
+            txn,
+            table="redactions",
+            values={
+                "event_id": event.event_id,
+                "redacts": event.redacts,
+                "received_ts": self._clock.time_msec(),
+            },
         )
 
     @defer.inlineCallbacks
@@ -1571,36 +1589,29 @@ class EventsStore(
         if self.hs.config.redaction_retention_period is None:
             return
 
-        max_pos = yield self.find_first_stream_ordering_after_ts(
-            self._clock.time_msec() - self.hs.config.redaction_retention_period
-        )
+        before_ts = self._clock.time_msec() - self.hs.config.redaction_retention_period
 
         # We fetch all redactions that:
         #   1. point to an event we have,
-        #   2. has a stream ordering from before the cut off, and
+        #   2. has a received_ts from before the cut off, and
         #   3. we haven't yet censored.
         #
         # This is limited to 100 events to ensure that we don't try and do too
         # much at once. We'll get called again so this should eventually catch
         # up.
-        #
-        # We use the range [-max_pos, max_pos] to handle backfilled events,
-        # which are given negative stream ordering.
         sql = """
-            SELECT redact_event.event_id, redacts FROM redactions
-            INNER JOIN events AS redact_event USING (event_id)
-            INNER JOIN events AS original_event ON (
-                redact_event.room_id = original_event.room_id
-                AND redacts = original_event.event_id
+            SELECT redactions.event_id, redacts FROM redactions
+            LEFT JOIN events AS original_event ON (
+                redacts = original_event.event_id
             )
             WHERE NOT have_censored
-            AND ? <= redact_event.stream_ordering AND redact_event.stream_ordering <= ?
-            ORDER BY redact_event.stream_ordering ASC
+            AND redactions.received_ts <= ?
+            ORDER BY redactions.received_ts ASC
             LIMIT ?
         """
 
         rows = yield self._execute(
-            "_censor_redactions_fetch", None, sql, -max_pos, max_pos, 100
+            "_censor_redactions_fetch", None, sql, before_ts, 100
         )
 
         updates = []
diff --git a/synapse/storage/events_bg_updates.py b/synapse/storage/events_bg_updates.py
index 6587f31e2b..5717baf48c 100644
--- a/synapse/storage/events_bg_updates.py
+++ b/synapse/storage/events_bg_updates.py
@@ -67,6 +67,10 @@ class EventsBackgroundUpdatesStore(BackgroundUpdateStore):
             self.DELETE_SOFT_FAILED_EXTREMITIES, self._cleanup_extremities_bg_update
         )
 
+        self.register_background_update_handler(
+            "redactions_received_ts", self._redactions_received_ts
+        )
+
     @defer.inlineCallbacks
     def _background_reindex_fields_sender(self, progress, batch_size):
         target_min_stream_id = progress["target_min_stream_id_inclusive"]
@@ -397,3 +401,60 @@ class EventsBackgroundUpdatesStore(BackgroundUpdateStore):
             )
 
         return num_handled
+
+    @defer.inlineCallbacks
+    def _redactions_received_ts(self, progress, batch_size):
+        """Handles filling out the `received_ts` column in redactions.
+        """
+        last_event_id = progress.get("last_event_id", "")
+
+        def _redactions_received_ts_txn(txn):
+            # Fetch the set of event IDs that we want to update
+            sql = """
+                SELECT event_id FROM redactions
+                WHERE event_id > ?
+                ORDER BY event_id ASC
+                LIMIT ?
+            """
+
+            txn.execute(sql, (last_event_id, batch_size))
+
+            rows = txn.fetchall()
+            if not rows:
+                return 0
+
+            upper_event_id, = rows[-1]
+
+            # Update the redactions with the received_ts.
+            #
+            # Note: Not all events have an associated received_ts, so we
+            # fallback to using origin_server_ts. If we for some reason don't
+            # have an origin_server_ts, lets just use the current timestamp.
+            #
+            # We don't want to leave it null, as then we'll never try and
+            # censor those redactions.
+            sql = """
+                UPDATE redactions
+                SET received_ts = (
+                    SELECT COALESCE(received_ts, origin_server_ts, ?) FROM events
+                    WHERE events.event_id = redactions.event_id
+                )
+                WHERE ? <= event_id AND event_id <= ?
+            """
+
+            txn.execute(sql, (self._clock.time_msec(), last_event_id, upper_event_id))
+
+            self._background_update_progress_txn(
+                txn, "redactions_received_ts", {"last_event_id": upper_event_id}
+            )
+
+            return len(rows)
+
+        count = yield self.runInteraction(
+            "_redactions_received_ts", _redactions_received_ts_txn
+        )
+
+        if not count:
+            yield self._end_background_update("redactions_received_ts")
+
+        return count
diff --git a/synapse/storage/schema/delta/56/redaction_censor2.sql b/synapse/storage/schema/delta/56/redaction_censor2.sql
new file mode 100644
index 0000000000..77a5eca499
--- /dev/null
+++ b/synapse/storage/schema/delta/56/redaction_censor2.sql
@@ -0,0 +1,20 @@
+/* Copyright 2019 The Matrix.org Foundation C.I.C.
+ *
+ * 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.
+ */
+
+ALTER TABLE redactions ADD COLUMN received_ts BIGINT;
+CREATE INDEX redactions_have_censored_ts ON redactions(received_ts) WHERE not have_censored;
+
+INSERT INTO background_updates (update_name, progress_json) VALUES
+  ('redactions_received_ts', '{}');