Well, I finally found the time to instal and configure vsftpd on my PI3. And the outcome is quite the opposite compare to FritzOS.
ftp transfer is way slower compared to SMB. Here’s the ranking.
1) SMB (3.5MB/s)
2) scp (2 … 2.5 MB/s)
3/4) ftp and sftp (1 … 1.5MB/s)
Maybe this data is of use for others.