No PCIe switch involved, every SSD is directly connected to the PCIe ports, the MB supports bifurcation. Observed on H11SSL-i and H12SSL-i motherboards with various models of SSDs (mostly Intel, Micron and Samsung). Windows defender switched off.
Hyper-V is on though. And I understand that when you switch on Hyper-V, even the host OS is really a VM on top of the hypervisor. So I wonder if that's not contributing (but disabling Hyper-V is not an option).
When you say 1.5GB/s was beatable, how do you measure it? Windows copy (or xcopy) or some benchmarking software?
There might be some confusion that’s holding you back on diagnostics though. If a PCIe switch was in the path and causing the issue then there’s no meaningful difference between “parallel” and “bidirectional.” The nature of the switch is that all the traffic goes through it and it has a top speed and that’s split among the operations. Read or write to a single drive gets full bandwidth, but copying from one to another is read and write, so each part gets 50%. Even on the same drive, write/read and write/read also get 50% each. Include other drives on the same switch and divide it again. “And” = divide.
Your platform makes that specific issue less likely, but hardware can be a bit more quirky than you might be giving it credit.
Of course you’ve turned off windows defender real-time scanning, but otherwise I can’t think of an on-by-default reason for Windows Server to cause that issue, but it isn’t inherent in the OS. I’ve used multiple versions in dozen GB/s arrangements - 1.5GB/s was beatable 20 years ago. There’s something going on with your settings or hardware. Good news is it might be fixable, bad news is you’ll have to go trekking around to find the issue. :/