MariaBackup very slow after upgrading from 10.2.14 to 10.3.8

We have a 3 node Galera cluster and a separate backup server where we run MariaBackup. The backup worked well with version 10.2.14. After upgrading the 4 servers to MariaDB / MariaBackup 10.3.8 the backup is much slower (as you can see in the attached log files).
On the backup server we run this from a script:

/usr/bin/ssh backup@$IP "/usr/bin/sudo /usr/local/bin/backup-mysql.sh" | mbstream -x -C $BACKUPDIR/$1/$WEEKDAY

/usr/bin/mariabackup --host=$IP --user=backup --password=xxxx --use-memory=10G --prepare --target-dir $BACKUPDIR/$1/$WEEKDAY

and this is called on the DB server:

/usr/bin/mariabackup --backup --use-memory=10G --galera-info --ftwrl-wait-threshold=40 --ftwrl-wait-query-type=all --ftwrl-wait-timeout=180 --kill-long-queries-timeout=20 --kill-long-query-type=all --user=backup --password=xxxx --stream=xbstream --target-dir /var/tmp