I've been using Unraid for about a year and a half now. My use is nothing exotic, just as a backup destination for several computers on my local network.
My array is currently 2 16 TB parity drives and 3 data drives (14, 14, 16). SATA interface.
Parity check takes about 50 hours for that array. The speed is about 85 Mbytes/sec. Is that about what you would expect?
Also, the array is about 75% full. Any guidance on how full it can get before it would be a good idea to add an additional data drive?