Getting same problem from a Debian 3.1 host to a Cisco PIX.
The infuratiating thing is that the problem comes and goes.

Thought it was an MTU problem but have tried different MTU's all the way along.
The tun device defaults to an MTU of 1390.

Its insane, the ssh response to the box on the other end is quite zippy, but large scp or rsync-via-ssh transfers just jam almost instantly.