UPD: Somehow, when running Windows 11 inside VM with raw vfio partition, I am getting full performance, which I could expect from raidz1 array. Maybe I am just testing speed on unraid incorrectly?
UPD2: Aha, I think I got it. My testing method was really bad. That was the only single-thread performance. I tried to run tests via fio utility -- and I got the numbers I expected for 12 threads. Sick!
fio --name=test --rw=read --size=1G --numjobs=12
> READ: bw=12.8GiB/s (13.7GB/s), 1092MiB/s-1410MiB/s (1145MB/s-1479MB/s), io=12.0GiB (12.9GB), run=726-938msec
fio --name=test --rw=write --size=1G --numjobs=12
> WRITE: bw=4625MiB/s (4849MB/s), 385MiB/s-404MiB/s (404MB/s-424MB/s), io=12.0GiB (12.9GB), run=2535-2657msec