I just wanted to do a followup on this that i think i just figured out my own problem. I changed the RAID-5 Stripe size from 128k (Asus default i guess) down to 64k and now my write performance is at 400-450MB/s versus the 32-50MB/s i was getting.
I am curious to know if you ever got this issue resolved. I have the exact same issue. I have a 1TB and 2TB Samsung 980 Pro nVME drives and a RAID-5 consisting of 3x12TB HGST drives and transferring files TO the NVME's is what i would expect but tra...