Performance question

Hello,

I'm using an "old" supermicro server as a NAS. The specs are:

Intel(R) Xeon(R) CPU W3690 @ 3.47GHz
24 GB Ram (I know, should be more)
LSI 9750-16i4e DISK
16x Seagate IronWolf 8 TB HDD 7200 U/Min, CMR, 256 MB Cache, SATA 6 Gb/s
They are in RAIDZ2. 41TB are in use 50TB are free.

I tried to do a performance test using dd: (i did execute this in the shell on the web interface, not sure if that matters)
dd if=/dev/zero of=/mnt/FMUS-NAS-04/test1.img bs=1G count=10

The result I get is...

Performance question