New MyBook Duo 4Tb astonishingly slow... I mean, really soul-numbingly slow

How are you connected to the drive?  Are you connecting through a router on your network?  I’ve read some of your older posts and you have been saying that you were connecting the network drive directly to your computers - both a pc and a mac. If that is so, then I’m amazed that you’re getting any transfer rate at all. 

I know TonyPH12345 told you it could be done, but you need to understand that that is not how that drive is designed to function. Now, if you are correctly setting it up on the network through a router, and you’re still getting those kinds of transfer rates, then you need to let us help you. You may have serious issues with that drive.  It may actually need to be replaced.