Is this really Pci-e 2.0? Judging by the speeds, it seems to work on Pci-e 1.1.
What does DiskInfo report?
@Dagal, you are right!
DiskInfo says: Transfer Mode: PCIe 1.0 x4 (current mode), PCIe 4.0 x4 (supported mode), but the board definitely supports PCIe 2.0. The thing is, this is the only PCIe slot, which recognizes the nvme on my mainboard 
I am using PCIe slot: PCIEX4_1 right now (seems like this slot only supports 1.0), PCIEX16_1 is the graphics card
I see I have reached the issue now you discussed above with Slava_1988…
That’s what I found for my board:
Expansion Slots: 4 x PCIe 2.0 x16 , support x16; x16/x16; x16/x8/x8 and x8/x8/x8/x8 configurations
1 x PCIe x4
1 x PCI 2.2