I ask this problem because I meet twice recently that the wal receiver process do not start after a long time.
first time:
I change recovery_min_apply_delay from default to 3d on a standby，the standby start but there is no receiver process，and on the master，pg_stat_replication show nothing.After 3 days the standby recover to normal,and I can see it in the pg_stat_replication on the master.
second time:
I change this 3day-behind-slave point to a cascade slave of the master，and the receiver process do not start too，the startup process replay the wal slowly.
I think this behavior for recovery_min_apply_delay is not good，because if the receiver do not fetch the wal for a long time(in these cases it must replay 3d's wal before wal receiver start)，the master will delete the wal，and the standby will need be re do.