I'm trying to use restic to backup ~3TB of data to another system using SFTP over a gigabit network connection.

I've set RESTIC_REPOSITORY and RESTIC_PASSWORT environment variables and used the following commands:
restic init
restic backup /mnt/disk

When I tried the latest release (0.15.1), I immediately noticed the slow transfer speed of around 10 MB/s. The cpu load was 60% idle most of the time.
First I tried a lot of different options:

• Disable backup progress estimation (--no-scan)
• Increasing backend connections (-o sftp.connections=10)
• Disable compression (--compression off)
• Increasing file read concurrency (--read-concurrency 8)
• Increasing pack size (--pack-size 64)
  None of these parameters did change anything.

Next I tried an older version from the Debian repository (0.11.0).
Just using this version caused my cpu to get maxed out and I reached about 60-70 MB/s transfer rate.

Then I tried every release from 0.11.0 to 0.15.1 and starting with 0.12.1 every version afterwards shows this issue.
I guess it must have something to do with a change that happened in 0.12.1.

It does only happen with SFTP repositories. A backup to a local repository is not affected.
I've also tried to init the repository with 0.12.0 (repository format version 1) and then doing the backup with the latest release. Also slow transfer.

Any ideas?