Using NFS v4.2 on Arch, nfs-utils 2.3.3, linux 4.18.16.
If a server has multiple nfs exports on different filesystems, and a
client moves a file from one of these exports to another, the file
contents transmit across the network.
If the file contents were kept on the server, there would be a great
speed boost. (Assuming the network speed is less than the server disk
I/O speed.)
Apologies if this is one of those things that sounds easy and has been
asked a lot but rejected.