I am currently running all my benchmarks with synchronous_commit=off and will get back with my findings.
It seems that PGOPTIONS="-c synchronous_commit=off" has a significant impact. However, I still can not understand why the TPS for the optimised case is LOWER than the default for higher concurrency levels!
[1] "Mostly default" settings are whatever ships with Ubuntu 18.04 + PG 11. A snippet of the relevant setts are given below:
max_connection=400
work_mem=4MB
maintenance_work_mem=64MB
shared_buffers=128MB
temp_buffers=8MB
effective_cache_size=4GB
wal_buffers=-1
wal_sync_method=fsync
max_wal_size=1GB
autovacuum=off # Auto-vacuuming was disabled
[2] An optimised version of settings was obtained from https://pgtune.leopard.in.ua/#/ and along with that the benchmarks were run with PGOPTIONS="-c synchronous_commit=off"