pg_basebackup inconsistent performance

We have pg_basebackup running on two of our DB servers that are replicated. We are running postgres 11, and it seems we are getting inconsistent performance from the backups and unsure as of why. We start out at 5-6 hours over a dedicated 10G port for 7TB. It creeps up to 8-9 hours then all of a sudden takes 12-16 hours. There seems to be no rhyme or reason for the extended backup times. The command we use for backups is as follows:

On server 2 (secondary), starts at 4PM
pg_basebackup --pgdata=/opt/postgres/pgbackup/`echo $DATE` --format=plain --write-recovery-conf --no-sync --wal-method=stream --checkpoint=fast --label=`hostname`-`echo $DATE` --no-verify-checksums --host=<server1> --username=replication --port=5432

On server 1 (Primary), starts at Midnight
pg_basebackup --pgdata=/opt/postgres/pgbackup/`echo $DATE` --format=plain --write-recovery-conf --no-sync --wal-method=stream --checkpoint=fast --label=`hostname`-`echo $DATE` --no-verify-checksums --host=<server2> --username=replication --port=5432

I'm not sure why or how we are running into the weeds. I am the SysAdmin and am not familiar with the inner workings of the DB. I can pass any commands that need run along to our DBA.

From the OS perspective, we are not seeing any problems with CPU, memory or disk. We are running on RHEL 7.7

Thanks!