Now I made new installation after few months but with 1 SSD. 918+ 6.2.2
Results are similar:
/$ dd if=/dev/zero bs=2M count=1024 | md5sum
1024+0 records in
1024+0 records out
2147483648 bytes (2.1 GB) copied, 20.6168 s, 104 MB/s
/$ dd if=/dev/zero bs=10M count=1024 | md5sum
1024+0 records in
1024+0 records out
10737418240 bytes (11 GB) copied, 80.3615 s, 134 MB/s
/$ sudo dd bs=5M count=256 if=/dev/zero of=/volume1/Share918/testx conv=fdatasync
256+0 records in
256+0 records out
1342177280 bytes (1.3 GB) copied, 4.18094 s, 321 MB/s
And CPU load is very low during that tests - maybe 17%