I've been dealing with the same problem for several years now. In the past, it wasn't a big deal for me because I didn't need the parity, but now, more and more data are on the array that I don't want to lose.
I'm not able to create a parity disk, even when I shut down Docker, remove the VM Manager, delete all plugins, and so on. First, I assumed there was a problem with the motherboard (ASUS ROG STRIX B550-F GAMING), so I updated the motherboard and checked all settings twice. Second, I assumed it was the Adaptec HBA because it was mentioned somewhere that an old firmware on the controller (Adaptec ASR-71605) itself has a problem, so I updated the controller.
When I just copy files between the disks, I get 10 to 20% iowait.
Maybe the controller will not work with Unraid or needs another driver.
I assume none of you have solved the issue, right?