First, let me address your concerns.
I'm copying from one slow drive, to another (different) slow drive. Blocking for reads on one drive until I write to another leaves a lot of room for improvement.
I developed a small cpp/h file that does this for me. It improved speed in my use case by ~40%. https://github.com/pauldotknopf/AsyncStreamCopyCPP/blob/master/examples/main.cpp
I'd like to know if there is something equivalent, but as a command. Preferably one that is similar to how the "cp" command operates.
# Use async read/write threads and shared/re-used buffers between the two. cp /slowdrive1/file1.txt /slowdrive2/file2.txt