bundles/backup-client: use --compress-level=1 instead of -z

bundles/backup-client/files/generate-backup

@@ -18,7 +18,8 @@ then
 fi
 
 do_backup() {
-    rsync -zaAP --numeric-ids --delete --relative \
+    # Compress level 1 is a good compromise between speed and cpu usage.
+    rsync --compress-level=1 -aAP --numeric-ids --delete --relative \
         --rsync-path="/usr/bin/rsync --fake-super" \
         -e "ssh -o IdentityFile=/etc/backup.priv -o StrictHostKeyChecking=accept-new -p ${port}" \
         "$1" "$ssh_login":backups/