Details:

Added PANIC to end of recovery so I could repeat this using identical WAL.

pg_ctl stop -D ../data -m smart
pg_ctl start -D ../data -l pg.log -w
psql -c "drop table if exists t1;" postgres > NUL
psql -c "create table t1 (a int primary key, b int not null) with (fillfactor = 85);" postgres > NUL
psql -c "insert into t1 select x,0 from generate_series(1,10000000) x;" postgres > NUL
psql -c "drop table if exists log_wal;" postgres > NUL
psql -c "create table log_wal (lsn pg_lsn not null);" postgres > NUL
psql -c "insert into log_wal values(pg_current_wal_lsn());" postgres > NUL
pgbench -n -f update.sql -t 3000000 -c 4 -j 4 -M prepared postgres
psql -c "select 'Used ' || pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), lsn)) || ' of WAL' from log_wal limit 1;" postgres
pg_ctl stop -D ../data -m immediate -w
echo Starting Postgres...
pg_ctl start -D ../data -l pg.log


2020-09-17 17:04:49.902 NZST [16496] LOG:  redo done at 0/E170FFD8 system usage: CPU: user: 74.21 s, system: 17.39 s, elapsed: 94.71 s
2020-09-17 17:06:46.685 NZST [4884] LOG:  redo done at 0/E170FFD8 system usage: CPU: user: 74.75 s, system: 16.56 s, elapsed: 94.56 s
2020-09-17 17:10:21.546 NZST [10272] LOG:  redo done at 0/E170FFD8 system usage: CPU: user: 73.62 s, system: 17.25 s, elapsed: 93.89 s

update_process_title = on

2020-09-17 17:12:45.246 NZST [14744] LOG:  redo done at 0/E170FFD8 system usage: CPU: user: 73.62 s, system: 17.07 s, elapsed: 93.88 s
2020-09-17 17:14:30.303 NZST [15692] LOG:  redo done at 0/E170FFD8 system usage: CPU: user: 73.50 s, system: 16.53 s, elapsed: 93.75 s
2020-09-17 17:16:10.040 NZST [3912] LOG:  redo done at 0/E170FFD8 system usage: CPU: user: 73.95 s, system: 16.65 s, elapsed: 93.80 s

L:\Projects\Postgres\install\bin>pgbench -S -T 60 postgres
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: simple
number of clients: 1
number of threads: 1
duration: 60 s
number of transactions actually processed: 560380
latency average = 0.107 ms
tps = 9339.576713 (including connections establishing)
tps = 9343.026904 (excluding connections establishing)


update_process_title = off
L:\Projects\Postgres\install\bin>psql -c "select pg_Reload_conf()" postgres
 pg_reload_conf
----------------
 t
(1 row)

L:\Projects\Postgres\install\bin>pgbench -S -T 60 postgres
starting vacuum...end.
transaction type: <builtin: select only>
scaling factor: 1
query mode: simple
number of clients: 1
number of threads: 1
duration: 60 s
number of transactions actually processed: 717872
latency average = 0.084 ms
tps = 11964.531821 (including connections establishing)
tps = 11969.342880 (excluding connections establishing)


postgres=# select (60 / (560380/717872.0)) - 60 as seconds_spent_setting_title;
NOTICE:  new title = SELECT
NOTICE:  new title = idle
 seconds_spent_setting_title
-----------------------------
     16.86270031050358685130
(1 row)

postgres=# select 16.86270031050358685130/560380.0 * 1000000 / 2 as microseconds_per_set_process_title;
NOTICE:  new title = SELECT
NOTICE:  new title = idle
 microseconds_per_set_process_title
------------------------------------
        15.045772788557395741500000
(1 row)

postgres=# select 15.045772788557395741500000*225 / 1000.0 as milliseconds_spent_setting_process_title_in_recovery;
NOTICE:  new title = SELECT
NOTICE:  new title = idle
 milliseconds_spent_setting_process_title_in_recovery
------------------------------------------------------
                           3.385298877425414041837500
(1 row)