Poor I/O performance on Asus Z790-P D4

[Quartz]

I have two NVME drives: a WD Black SN750 and a Corsair MP400 and I am getting about 300 MB/s when transferring data between them - both ways - when I should be getting 10 times that, and 10 Gb/s on my LAN connection. The test dataset is 110 GB.

I've just upgraded to an i7-13700 in a Z790-P D4, transferred the drives, updated all drivers, updated the BIOS, and reactivated Windows. The NVME drives are in slots M2_1 and M2_2. I have a RTX 4090 in the x16 slot and an Intel X550 NIC in the bottom x4 slot. BIOS is up to date. There's a 1TB NVME cache drive on the NAS.

I'm seeing high CPU activity on core 8 of my 13700 but have not managed to isolate the process.

 

[Quartz]

The SN750 is 500 GB and the MP400 is 4 TB. I thought about the cache issue but I read on overclock3d.net that they had to write 750 GB to their MP400 before overwhelming the cache - not that it had 750 GB of cache but the cache worked well enough that it coped with 750 GB. As for the NAS, I managed a solid 500 MB/s (5 Gbps) running off a SATA SSD in a different PC.

 

[Quartz]

If you use a smaller dataset, e.g. 10GB, do you still only get 300 MB/s?

No. I tried copying a 4 GB folder to my NAS and it quickly went to 400 MB/s but subsided to a steady 315 MB/s. Copying between drives seems odd. I tried copying a 20 GB dataset and speeds would gradually rise to about 500 GB/s but then fall back to very low speeds, down as low as 11 MB/s. It's a very distinctive saw-tooth pattern.