scp troughput

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

[This may belong to tech-userlevel@]

I just observed that scp throughput between my NetBSD machines stalls at some 
20 MB/s. Raw troughput (iperf) is up to 800 Mbit/s. CPUs are nowhere near 
saturated. The weird thing it always saturates at nearly the exact same 
level independent of hardware and file size.

If I scp to a Debian machine, I get 65 MB/s, CPU bound on the receiving side.
Debian->NetBSD is slow, Debian->Debian is fast.

This reminds me of some Nagle-related problem but scp claims to have set 
TCP_NODELAY.

Any hints?