Lag clarification with Sync Replication

I have made a synchronous replication setup with PG12.
During data loading of 200 gig in master node, I found that there is a lag
that it shows on the slave. But, ideally for sync replication, lag should
be always 0 right.
Because commit never happens in master before updating the slave.
log_delay
------------
871.549671
(1 row) log_delay
------------
871.557246
(1 row) log_delay
-----------
0
(1 row) log_delay
-----------
0
(1 row)

*Query Used for checking Standby Replication Lag:*
psql -d perfbench -p 5432 -c "SELECT CASE WHEN pg_last_wal_receive_lsn() =
pg_last_wal_replay_lsn()
THEN 0
ELSE EXTRACT (EPOCH FROM now() - pg_last_xact_replay_timestamp())
END AS log_delay;"

*Setting Used in PG12*

cat postgresql.conf |grep synchronous_

synchronous_commit = remote_write # synchronization level;

synchronous_standby_names = 'ANY 1 (standby1,standby2)' # standby
servers that provide sync rep

cat postgresql.conf |grep primary_conn

primary_conninfo = 'user=edbrepl host= 10.12.11.101 port=5432
application_name=standby1 '

Thanks,
Raj Kumar