I compared transfer speed between 2 local machines. one file of 120MB of random data. with scp it transfered at approximately 12MB/s, so it was done in approximately 10-15 seconds (did it several times). With rsync it was done in approximately 20 seconds. With fish (in dolphin) it took about 1'15".

On one hand I'm running Sid and KDE4, on the other hand it's fedora with gnome but I can't tell exact version number.

Before that I had always assumed that fish, rsync and scp were relying on the same transfer mechanism, especially scp and fish: transfer data over a ssh connection.

Is it because FISH implements lots of features not available in SCP resulting in huge overhead? Is it a know issue? Could it be sped up by falling back to a scp-like transfer under some conditions?

fish uses console commands and a perl server side script to work. It does not require any special support to be enabled in the SSH server. SFTP on the other hand needs the appropriate extension enabled to work ( which it is in virtually all cases )

This is a case of workaround vs. something designed for the task.