[Samba] Odd, slow Windows XP client download behavior

[Chris]

I recently set up a Samba server (3.0.24-6etch9) on my debian etch system.
While accessing its shares through 1GBit Ethernet with a Windows XP SP2
client there's some real odd behavior:

In one direction (uploading files from WinXP -> debian) the speed is just
great, no matter what, around 35MiB/s.
In the other direction (downloading a file WinXP <- debian) it idles at
around 120KiB/s which is, of course, unbearable.
Now, interestingly, when I have an upload running the former slow download
speed just explodes and is right up there at 20MiB/s (with the upload
dropping to 20MiB/s). If I stop the upload, the download retreats back to
120KiB/s instantly.

Btw, the FTP speed is fine in both directions, 30 - 50 MiB/s.

Playing around with the global settings in smb.conf brought no relief
whatsoever (TCP_NODELAY, SO_SNDBUF etc) and a different samba version
(3.0.26a-18) didn't help either.

Then, I had a friend bring over his Windows Vista machine, which accessed my
Samba share without any of the above described problems at > 30 MiB/s in
both directions.
So I decided my WinXP machine must be the main problem.

Today I thought I found the solution but it was merely a step in the right
direction (if even that):
When I disable OpLocks in the Windows Registry according to Windows KB
article 296264 (disabling them in the shares part of smb.conf has the same
effect), the download speed turns out to be 35MiB/s. But the performance is
still not satisfiable: The download speed reaches the 35MiB/s only after
some sort of delay which proved to be quite annoying because it slows down
executing files, playing movies, skipping through movies, etc. considerably.
Now again, the interesting part: This delay doesn't occur at all, when I
have an upload running at the same time. Files open instantly, I can skip
through a movie like it was on my local hard drive, etc.

Now I don't know what else to try to get my WinXP to communicate correctly
with the samba server, do you have any suggestions?

Kind regards,
Chris