I am having a similar issue, my writes are hovering around 20MB/sec, but my reads are around 2MB/sec, bursting up to 7MB/sec at times. If I copy the same file to my Windows 10 box, it is a sustained 15MB/sec for the entire process.
Is anyone else having a problem since Calibre 4.0 came out? After I log in, I just get a white screen. Nothing else. I also tried the RDP-Calibre container and the same issue. The logs show I am authenticated then nothing. Just nothing.