Innobackupex taking huge time for preparing backup

We have a database on PXC which runs into 3 TB in size and growing. For backup we choose xtrabackup (innobackupex). We are taking backup to a USB based external HDD. The backup took about 9 hours to complete. But, when we are trying to prepare the backup, it is has only completed about 50 % in 24 hours time. I am preparing the backup as below

innobackupex --apply-log --redo-only --use-memory=4G /PATH_TO_BACKUP

Is this slowness expected ? It took only 9 hours to take full backup to the same disk, but only prepared 50% in 24 hours. Am I missing something ?