Encrypting of Time Machine takes extremely long

Just configured a new MacBook Pro 2017 with 1 TB SSD and use a new USB-c LaCie 4 TB drive, High Sierra 10.13.3. Partitioned, formatted and encrypted the drive via DiskUtility. The TimeMachine backup took quite some time to backup the 650 GB but then it started 'Encrypting Backup Disk. It took ages, after this I followed all suggestions on this forum, I did it all again; (format partition with encryption, TM Backup, etc.). but without any effect unfortunately: it is Encrypting now for 58 hours and is on 24%. The MacBook is connected to the power adapter, the option 'Prevent computer from sleep automatically when display is off' is on and 'Put hard disks to sleep when possible' is off. The little light on the HD is constantly flashing.

Any one an idea how to speed it up or have an alternative how to encrypt a backup drive?