diff --git a/changelog.d/4798.misc b/changelog.d/4798.misc
new file mode 100644
index 000000000..d60f208dc
--- /dev/null
+++ b/changelog.d/4798.misc
@@ -0,0 +1 @@
+Add some debug about processing read receipts.
diff --git a/synapse/storage/receipts.py b/synapse/storage/receipts.py
index 0ac665e96..0fd1ccc40 100644
--- a/synapse/storage/receipts.py
+++ b/synapse/storage/receipts.py
@@ -346,15 +346,23 @@ class ReceiptsStore(ReceiptsWorkerStore):
 
     def insert_linearized_receipt_txn(self, txn, room_id, receipt_type,
                                       user_id, event_id, data, stream_id):
+        """Inserts a read-receipt into the database if it's newer than the current RR
+
+        Returns: int|None
+            None if the RR is older than the current RR
+            otherwise, the rx timestamp of the event that the RR corresponds to
+                (or 0 if the event is unknown)
+        """
         res = self._simple_select_one_txn(
             txn,
             table="events",
-            retcols=["topological_ordering", "stream_ordering"],
+            retcols=["stream_ordering", "received_ts"],
             keyvalues={"event_id": event_id},
             allow_none=True
         )
 
         stream_ordering = int(res["stream_ordering"]) if res else None
+        rx_ts = res["received_ts"] if res else 0
 
         # We don't want to clobber receipts for more recent events, so we
         # have to compare orderings of existing receipts
@@ -373,7 +381,7 @@ class ReceiptsStore(ReceiptsWorkerStore):
                         "one for later event %s",
                         event_id, eid,
                     )
-                    return False
+                    return None
 
         txn.call_after(
             self.get_receipts_for_room.invalidate, (room_id, receipt_type)
@@ -429,7 +437,7 @@ class ReceiptsStore(ReceiptsWorkerStore):
                 stream_ordering=stream_ordering,
             )
 
-        return True
+        return rx_ts
 
     @defer.inlineCallbacks
     def insert_receipt(self, room_id, receipt_type, user_id, event_ids, data):
@@ -466,7 +474,7 @@ class ReceiptsStore(ReceiptsWorkerStore):
 
         stream_id_manager = self._receipts_id_gen.get_next()
         with stream_id_manager as stream_id:
-            have_persisted = yield self.runInteraction(
+            event_ts = yield self.runInteraction(
                 "insert_linearized_receipt",
                 self.insert_linearized_receipt_txn,
                 room_id, receipt_type, user_id, linearized_event_id,
@@ -474,8 +482,14 @@ class ReceiptsStore(ReceiptsWorkerStore):
                 stream_id=stream_id,
             )
 
-            if not have_persisted:
-                defer.returnValue(None)
+        if event_ts is None:
+            defer.returnValue(None)
+
+        now = self._clock.time_msec()
+        logger.debug(
+            "RR for event %s in %s (%i ms old)",
+            linearized_event_id, room_id, now - event_ts,
+        )
 
         yield self.insert_graph_receipt(
             room_id, receipt_type, user_id, event_ids, data