postgresql streaming replication slow on macOS - postgresql

Question

I am using PostgreSQL 10.1 on MAC on which I am trying to set up streaming replication. I configured both master and slave to be on the same machine. I find the streaming replication lag to be slower than expected on mac. The same test runs on a Linux Ubuntu 16.04 machine without much lag.
I have the following insert script.
for i in $(seq 1 1 1000)
do
bin/psql postgres -p 8999 -c "Insert into $1 select tz, $i * 127361::bigint, $i::real, random()*12696::bigint from generate_series('01-01-2018'::timestamptz, '02-01-2018'::timestamptz, '30 sec'::interval)tz;"
echo $i
done
The lag is measured using the following queries,
SELECT pg_last_wal_receive_lsn() - pg_last_wal_replay_lsn();
SELECT (extract(epoch FROM now()) - extract(epoch FROM pg_last_xact_replay_timestamp()))::int;
However, the observation is very unexpected. The lag is increasing from the moment the transactions are started on master.
Slave localhost_9001: 12680304 1
Slave localhost_9001: 12354168 1
Slave localhost_9001: 16086800 1
.
.
.
Slave localhost_9001: 3697460920 121
Slave localhost_9001: 3689335376 122
Slave localhost_9001: 3685571296 122
.
.
.
.
Slave localhost_9001: 312752632 190
Slave localhost_9001: 308177496 190
Slave localhost_9001: 303548984 190
.
.
Slave localhost_9001: 22810280 199
Slave localhost_9001: 8255144 199
Slave localhost_9001: 4214440 199
Slave localhost_9001: 0 0
It took around 4.5 minutes for a single client inserting on a single table to complete on master and another 4 minutes for the slave to catch up. Note that NO simultaneous selects are run other than the script to measure the lag.
I understand that replay in PostgreSQL is pretty simple like, "move a particular block to a location", but I am not sure about this behavior.
I have the following other configurations,
checkpoint_timeout = 5min
max_wal_size = 1GB
min_wal_size = 80MB
Now, I run the same tests with same configurations on a Linux Ubuntu 16.04 machine, I find the lag perfectly reasonable.
Am I missing anything?