Update: Slow parity check is a symptom of a 5.8 Kernel Bug where pre-Skylake CPUs get stuck at the minimum pstate MHz (often 800MHz). Scroll down in this thread for links to the associated Kernel bug and discussion. Otherwise, this link will jump you to my updated post in this thread.
I've been noticing that 6.9.0 has been slower than 6.8.3 for disk access when using dockers (also the high amount of disk reads on my cache even after making them 1MB aligned, but that's another issue).
I did my first parity check and it's markedly slower than historical. Normally parity check will complete in under 19hours at 118MB/s. The current parity check run happening now has been running for 9.5 hours and is only 30% done at 74 MB/s - with another 21 hours estimated to remain.
If that estimate is true (and I know things slow down toward the end), that'll be over 30 hrs vs 19 hrs. 10 more hours than history shows. I haven't made any hardware changes vs 6.8.3.
I've attached my diagnostics as well.