BUG #15609: synchronous_commit=off insert performance regression with secondary indexes

The following bug has been logged on the website:

Bug reference: 15609
Logged by: Jean Paolo Saul
Email address: paolo(dot)saul(at)verizonconnect(dot)com
PostgreSQL version: 11.1
Operating system: CentOS Linux release 7.6.1810 (Core)
Description:

Summary:
We are considering upgrading to PG11 and during performance testing we
have found that
PostgreSQL 11 inserts are around 25% slower than inserts in PostgreSQL
9.5.

Tools Used:
pgbench (11.1)

Test Overview:
1) InitDB and start four instances using versions PG9.5.15, PG9.6.11
PG10.6, and PG11.1
2) Create a test table
3) pgbench using inserts to the test table
3.1) test using default config settings , synchronous_commit=off ,
fsync=off
3.1.1) test with primary key only , primary key with one secondary
index , primary key with two secondary indexes , primary key with three
secondary indexes

Test Setup:
Amazon EC2 Instance:
m4.16xlarge - 64 cores, 251GB RAM
50GB EBS, volume type: io1

Table:
CREATE TABLE test_indexes (test_indexes_id BIGSERIAL, bool_data BOOLEAN,
int_data INTEGER, text_data TEXT, PRIMARY KEY (test_indexes_id))

Indexes:
CREATE INDEX bool_idx ON test_indexes (bool_data)
CREATE INDEX int_idx ON test_indexes (int_data)
CREATE INDEX text_idx ON test_indexes (text_data)

Test Results (TPS is average of three runs):
** DEFAULT CONF VERSION TPS DIFF FROM PG95
pkey only
PG9.5 42414 0.0%
PG9.6 41967 -1.1%
PG10 43443 2.4%
PG11 43676 3.0%
bool index
PG9.5 42310 0.0%
PG9.6 42082 -0.5%
PG10 41902 -1.0%
PG11 42305 0.0%
bool+int index
PG9.5 41539 0.0%
PG9.6 41966 1.0%
PG10 41294 -0.6%
PG11 41819 0.7%
bool+int+text index
PG9.5 40000 0.0%
PG9.6 40526 1.3%
PG10 40582 1.5%
PG11 39882 -0.3%

** SYNCHRONOUS_COMMIT=OFF VERSION TPS DIFF FROM PG95
pkey only
PG9.5 103904 0.0%
PG9.6 100017 -3.7%
PG10 103857 0.0%
PG11 117147 12.7%
bool index
PG9.5 67283 0.0%
PG9.6 70850 5.3%
PG10 51113 -24.0%
PG11 49659 -26.2%
bool+int index
PG9.5 66048 0.0%
PG9.6 68247 3.3%
PG10 50558 -23.5%
PG11 47734 -27.7%
bool+int+text index
PG9.5 66732 0.0%
PG9.6 67131 0.6%
PG10 47157 -29.3%
PG11 47692 -28.5%

** FSYNC=OFF (10 SECS) VERSION TPS DIFF FROM PG95
no secondary index
PG9.5 90974 0.0%
PG9.6 90174 -0.9%
PG10 93661 3.0%
PG11 101758 11.9%
bool index
PG9.5 65328 0.0%
PG9.6 68447 4.8%
PG10 45757 -30.0%
PG11 46610 -28.7%
bool+int index
PG9.5 63247 0.0%
PG9.6 64010 1.2%
PG10 43378 -31.4%
PG11 45467 -28.1%
bool+int+text index
PG9.5 60768 0.0%
PG9.6 63230 4.1%
PG10 40968 -32.6%
PG11 44017 -27.6%

Questions:
Is there an extra setting for Postgres 10+ required to "recover" the
performance loss from PG9.5?
We are using PG9.5 with synchronous_commit=off in production and
majority of our tables have secondary indexes.
Why is PG10+ slower by default when synchronous_commit is off?

Notes:
Tested with all wal_sync_methods: fdatasync, open_datasync, fsync,
fsync_writethrough(fails), open_sync, with no statistical significance
found
Did not test with updates or deletes