I do bump up maintenance_work_mem temporarily during a restore.
it helps in rebuilding on indexes a little faster.
Turning fsync off during restore will help the restore a little fast too but in case of any crash you may have to restart the restore from scratch.
Also do have the option to take pg_dump and run pg_restore ? or you just the have the raw sql dump to work with?
if you have the option of taking a dump again, you can try using pg_dump and pg_restore with -Fc (custom format) and -j n (parallel) option along with temp bump in maint memory.
This will make the restore a little faster that raw sql dump I think.
If you are on pg10 or above? you can use logical replication to mirror the database.
There are blogs by several people explaining how to do that, that may be helpful.
IMO you are using the slowest tool to import.
Just one quick question: Why can't you take cluster backup using any of the tools available and then drop all
unwanted databases after you import the cluster.
pg_basebackup will do a good job.
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