Pgbackrest backup is too slow

Hello,
I have a Centos 7 server which runs Postgresql 10.7. I am using pgbackrest
to take db backup.
Problem is backup is too slow.
My data dir size is 9.6G and full backup runtime is 22 mins
I also tried using process-max=3, full backup runtime = 21 mins
Following are the backrest configurations:
backrest.conf for pg-primary:
[data]
pg1-path=/var/lib/pgsql/10/data

[global]
log-level-file=detail
repo1-path=/var/lib/pgbackrest
repo1-retention-full=2
start-fast=y
process-max=3

Am I doing something wrong here?
In production we have a database of size more than 500GB. It would not be
feasible for me to use it as it is.pgbackrest version is
2.18-1.rhel7.x86_64.
installed from pgdg yum repo.
Also I need to take a backup from the remote machine which would also
increase the overall runtime of backup/restore.

Thanks for the help in advance

Regards,
Ajay pratap

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