New 2TB portable hard drive has ~2MB/s read and write speeds
Solution 1:
So, the problem has gone away after doing a quick format in Windows 10. Performance has jumped up to around 105MB/s reads and writes, and it's sustained even after copying hundreds of GB.
I don't have any explanation why doing this should have altered the read and write speed of the drive. It was a brand new drive. And, many of the ways I had tested read/write speed were accessing the raw drive and bypassing the formatting anyway (eg, dd).
But since it's appropriate to give the solution I found for myself, here is my answer, such as it is.