Is your nvme slot running in full x4 mode or in x2 mode? Some motherboards have nvme slots that share bandwidth with other components such as the sata ports
Above is a good explanation as reported max read speed is half a PCI 3 NVMe slot (approx x2 out of 4)
Check slot/port and BIOS PCI config. Rearranging devices may help.
Also to affirm prev points:
SN850 needs PCI4 x4 to go over 3 GB/s.
SN750 is PCI 3 x4, so never over 3.
Samsung 960 has PCI 3 but flash may not keep up over long haul. That design era was on cusp of full PCI 3 perf when popular
The Samsung 960 PRO M.2 SSD is the newest addition to the company’s portfolio…
www.storagereview.com
//In our 2MB sequential benchmark, the Samsung 960 Pro recorded some best-of-class results with a blistering 2,222.93MB/s read and 1,786.72MB/s write. The next best drive was the OCZ RD400 m.2 NVMe, which posted 1,934.44MB/s read and 1,475.39MB/s write.//
Pre Intel 11th gen, no PCI 4, so single NVMe capped at 3 GB/s — AMD has offered PCI 4 in some chips for a couple years, but AMD users see same situation because PCI 4 NVMe drives are very recent.