A couple of people here mentioned not to use Storage Spaces Parity; yes you are correct if you use it without any SSD or NVMe cache, but if you do it works incredibly well.
I have 6 x HGST 10TB drives in a Parity volume with 2 x 250GB Samsung 850 Pros mirrored for the cache.
From the internal SATA M.2 to the new ReFS volume I was getting sustained transfer speeds of just under 400mb/s.
Also, ReFS dedupe support in Server 2019 works a treat!