diff --git a/changelog.d/6148.misc b/changelog.d/6148.misc
new file mode 100644
index 0000000000..1d5213345c
--- /dev/null
+++ b/changelog.d/6148.misc
@@ -0,0 +1 @@
+Improve performance of `find_next_generated_user_id` DB query.
diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index 241a7be51e..1a859352b6 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -493,7 +493,9 @@ class RegistrationWorkerStore(SQLBaseStore):
         """
 
         def _find_next_generated_user_id(txn):
-            txn.execute("SELECT name FROM users")
+            # We bound between '@1' and '@a' to avoid pulling the entire table
+            # out.
+            txn.execute("SELECT name FROM users WHERE '@1' <= name AND name < '@a'")
 
             regex = re.compile(r"^@(\d+):")