diff --git a/synapse/storage/schema/transactions.sql b/synapse/storage/schema/transactions.sql
index 86f530d82..2d30f99b0 100644
--- a/synapse/storage/schema/transactions.sql
+++ b/synapse/storage/schema/transactions.sql
@@ -42,6 +42,7 @@ CREATE INDEX IF NOT EXISTS sent_transaction_dest ON sent_transactions(destinatio
 CREATE INDEX IF NOT EXISTS sent_transaction_dest_referenced ON sent_transactions(
     destination
 );
+CREATE INDEX IF NOT EXISTS sent_transaction_txn_id ON sent_transactions(transaction_id);
 -- So that we can do an efficient look up of all transactions that have yet to be successfully
 -- sent.
 CREATE INDEX IF NOT EXISTS sent_transaction_sent ON sent_transactions(response_code);
diff --git a/synapse/storage/transactions.py b/synapse/storage/transactions.py
index 9d14f8930..e06ef3569 100644
--- a/synapse/storage/transactions.py
+++ b/synapse/storage/transactions.py
@@ -19,8 +19,6 @@ from collections import namedtuple
 
 from twisted.internet import defer
 
-from synapse.util.logutils import time_function
-
 import logging
 
 logger = logging.getLogger(__name__)
@@ -200,7 +198,6 @@ class TransactionStore(SQLBaseStore):
             self._get_transactions_after, transaction_id, destination
         )
 
-    @time_function
     def _get_transactions_after(cls, txn, transaction_id, destination):
         where = (
             "destination = ? AND id > (select id FROM %s WHERE "
@@ -231,7 +228,6 @@ class TransactionStore(SQLBaseStore):
             "get_destination_retry_timings",
             self._get_destination_retry_timings, destination)
 
-    @time_function
     def _get_destination_retry_timings(cls, txn, destination):
         query = DestinationsTable.select_statement("destination = ?")
         txn.execute(query, (destination,))