Using scp with large files kills ethernet?
Turns out: the culprit was neither the Mac itself, nor my Linux box.

I later found that the LogiLink USB-C to Ethernet adapter caused this. 

And for the record - there were various other symptoms, such as: 


download of large files in my browser being extra slow
replicating large files via IBM Notes not only being slow, but basically turning the whole machine into "almost unresponsive" mode