very slow drbd replication

SLES 12 SP2 + online updates.

Its a 1 TB volume with XFS file system and both the nodes are connected via 1 Gbps network. DRBD replication started 41 hours before but it just reached 8.78 %.
drbdsetup status oracle --verbose --statistics
oracle node-id:1 role:Primary suspended:no
  volume:0 minor:0 disk:UpToDate
      size:1904549052 read:515556707 written:13288947 al-writes:6734 bm-writes:0
      upper-pending:0 lower-pending:0 al-suspended:no blocked:no
  prdnode2 node-id:0 connection:Connected role:Secondary congested:no
    volume:0 replication:SyncSource peer-disk:Inconsistent done:8.78
        received:0 sent:176786480 out-of-sync:1737341808 pending:0 unacked:0

Why replication is so slow ?

Also would you advise me to run the following command on SyncTarget node:

        drbdadm disk-options --c-plan-ahead=0 --resync-rate=110M oracle

and then later on edit the resource file to add the following:

     disk { resync-rate 100M; }

current configuration:
global { usage-count no;}

common {
	handlers {  fence-peer "/usr/lib/drbd/"; after-resync-target "/usr/lib/drbd/"; }

	startup { wfc-timeout 0; degr-wfc-timeout 120;  }

	disk {   on-io-error detach; 	al-extents 3389; }
	net { after-sb-0pri discard-zero-changes;  after-sb-1pri discard-secondary; 	max-buffers 8000;                 
    		max-epoch-size 8000; 	sndbuf-size 0;    verify-alg md5; ping-int 2; ping-timeout 2; connect-int 2;
    		timeout 5;  ko-count 5; }

resource file:
resource oracle   {
   device /dev/drbd_r0 minor 0;
   meta-disk internal;

   on node1 { disk "/dev/sdb2"; address; }  

   on node2 { disk "/dev/sdb2";  address; }


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