while not as bad as your I have similar problems, from my pc I get 11MB/s over 100mbit lan, from my laptop less than 5MB/s, using the same cable, same files.
over wireless I get 6MB/s on desktop and 2.5MB/s on laptop but this is more comphrensible as one got 150mb/s wifi card while the other only 54g.
the most awkward thing is that I get less than 8mb/s while copying between the two disks inside my 320l (over ssh shell), really strange that it gets lower internal transfer rate than over ethernet.