abysmally low scp performance

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I'm just scp-ing a multi-gigabyte file from one NetBSD-8 machine to another, 
and get a whopping 10MB/s throughput.
Interfaces are wm, 1000BaseT (says ifconfig and the switch), connected to 
the same (enterprise) switch.
Discs are ~20% busy on the receiving end, less on the sending end.
ssh/sshd are at ~20% CPU max, CPUs are 50% (sending side, which does multiple 
other things)/80% (receiving side, which does nothing) idle.

Any hints?