See also Caution on macOS 10.12.4: sudo is broken.
Today, home from a trip, I needed to transfer 40GB or so of image files between my travel laptop (2015 MacBook Pro running 10.12.4) and my home computer (2013 Mac Pro running 10.12.3).
First, I connect to the Mac Pro, and started copying to the Mac Pro from the MacBook Pro (Mac Pro acting as the server). Data rates over gigabit ethernet were around 2 kilobyte per second, eventually resulting in an I/O error (-36).
Next I tried copying to the Mac Pro from the MacBook Pro (MacBook Pro acting as the server). Same deal—a few kilobytes per second.
Both machines had been freshly booted and otherwise show normal network speed. Never before have I see such broken network behavior.
In the end, I was forced to copy the files to a drive, then move the drive to the Mac Pro and copy the files to it—file sharing is unusable between the two.