cbbackupmgr is very slow at taking an initial backup on a cluster that has ~550 M docs

Build : 5.0.0-3064

Cluster Info :
Single node
RAM=128 GB
40 core CPU
4 buckets (3 standard 1 ephemeral)

Bucket Name / No. of Items / Resident Ratio
default / 224504072 / 85%
other-1 / 152678557 / 93%
other-2 / 152784699 / 93%
ephemeral / 3368211 / 93%

In ~20 mins, only ~9.5 M items could be backed up.

root@espresso:/opt/couchbase/bin# ./cbbackupmgr config -a /disk2/backup -r backup
Backup repository `backup` created successfully in archive `/disk2/backup`
root@espresso:/opt/couchbase/bin# ./cbbackupmgr backup -c 172.16.12.9 -u Administrator -p password -a /disk2/backup -r backup

Backing up to 2017-06-08T22_00_00.467963475+05_30
Copying at 2.06MB (about 4h 47m 36s remaining) 9452820 items / 9.46GB
other-1 [========== ] 6.21%

Will attach the backup logs once the backup process is completed.