Other than that, if the standbys are slow, disconnected, or down, the primary will wait for acknowledgements from one or more synchronous standbys. By resetting the synchronous_standby_names parameter, you changed the configuration of which standby servers need to confirm transactions for them to be considered committed. Please execute the following command on the primary node after resetting the synchronous_standby_names to reload the latest configurations:
SELECT pg_reload_conf();
Muhammad Affan (아판)
PostgreSQL Technical Support Engineer / Pakistan R&D