I had a similar case. I resolved it by using msconfig did not load non-microsoft programs AND after Hiding Microsoft Services, I disabled all services. Then it DID NOT SLOW down. I enabled the services in a controlled manner to determine the OLD Western Digital were the problem. I uninstalled all my WD software and installed the latest and the problem was gone.
See if you have similar services that cause your problem.
ALSO, when connected through my docking station, Read/Write data wate was 424MB/s. When Connected DIRECTLY to my TB3 port on the PC (Which is connecting to the docking station when I use it), the datarate doubled up to about 800MB/s.
So make sure you check the speed on the native PC and not via docking, hugs or other.
Good luck,
Mickey
