You're comparing apples with oranges. Different CPU models will have different delays.
Also I meant change the NVMe drives as apart from make, model and revision you'll have different workloads in Proxmox configured. The IO delay should decrease if the CPU is busy, but this comes back to what the metric is measuring. If it were above say 20% then you might have a hardware issue, probably a configuration issue though.
I am not saying you should keep the machine if you are unhappy with it I just feel your conclusions need more evidence and I think the seller offering a refund is a positive outcome. I'm not clear on distance selling applying globally but I think it does and you could press them to pay the return charges if you're inside the distance selling window.
I compared an N5105 to an N6005, not to a Xeon, an AMD, or a Core processor. They share the same architecture (and same motherboard), hence my doubts on the BIOS configuration (that's probably where the issue lies).
If I had an IO delay of 20% I would have returned the units already. I swapped a different NVME drive in and the results are exactly the same, while idling a fresh install the IO delay sits at 1.5%.
While you consider a positive outcome, I consider it a 'meh'. In my opinion a positive outcome on a product is to try to fix it, instead of saying 'if you're not happy feel free to ship it back'.
If humanity applied the same principle we would still be at Neanderthal age.
To wrap it up, let's agree to disagree here.