100MB/s or less? Are you using SMR 5400rpm disks? I routinely get 180-220MB/s throughput per Exos drive.
dd if=/dev/zero of=tmp.img bs=1G count=1 oflag=dsync
The drive appears to be - WDC WD160EDGZ-11B2DA0, it's a 16TB disk, which I thought would be SMR, due to its size, but maybe I'm wrong.
Just found https://www.reddit.com/r/DataHoarder/comments/qf5ar3/is_this... which seems to indicate it's CMR.
This will reduce performance