I have exactly similar problem.
I have motherboard ASUS TUF GAMING X570-PLUS.
There are two m.2 slots, which according to specs, are PCIE 4.0 x4.
When i put disk into m2.1, i got info, that connection is detected as PCIE 3.0 x1, and then i have trasnfers around 800 MB
When i put disk into m2.2 i got info, that connection is detected as PCIE 3.0 x4, and then i have transfers around 1700 MB (very similar to mentioned in this topic).
Sn750 is connected as “storage” drive. I have second, sata ssd disk (128 gb) which boots Windows 10.
I tried to update bios and amd chipset drivers to newest one. The same goes to sn750 - it is up to date.
What should i do to fully enjoy sn750 power?
Does anyone can help me?