just bought a usbc-sata-v dock. download speeds are acceptable, in the range of 180-200mbytes/sec. upload speeds are too slow, around 45 mbytes/sec. I have tried this unit on 2 different computers ( both running windows 10 x64) with 3 different drives. I have used both the dual usb-c and the usb-c to legacy usb connector - no difference. I own an older plugable usb3-sata-u3 dock and have no such issues.
hmmm…
While experimenting with this, I discovered that if I reboot/power on windows 10 with the dock powered on and connected, upload speeds are what i expect, around 150-170MBs. If i connect / power up the dock with windows 10 already running, upload speeds never get above 45MBs. So, issue is resolved.