Soldato
Might be this: https://www.pcgamer.com/uk/wd-is-fi...ack-sn850-ssds-run-slow-on-x570-motherboards/
"The folks at ComputerBase ran some tests and observed write performance being cut nearly in half when connecting the drive to the aforementioned slot—one of the benchmarks saw write speeds go from 5,255MB/s to 2,972MB/s, which is a 43.4% reduction."
Firmware fix was posted in July: https://www.tomshardware.com/uk/news/new-sn850-firmware-restores-amd-x570-performance-loss
Absolutely sure you are on latest firmware?
Just double checked and its running the latest firmware but says says the connection is Gen4 but only 2 lanes. Didn't think anything reduced the lanes on X570 so I'll double check settings in bios later