The host is running Windows XP Prof SP3 and the client is running Windows 7 Ultimate SP1 x64.
So ... I connect to the remote machine with a 15mbps of ADSL. Which ... I guess, got enough downstream. (~1.3 MB/s). The host machine got a 35mbps of upload. Which is again, fast enough to pump the data with max speed.
All I get is like ~320-400 kbps transfer speed. That is really slow. Is there a way to speed it up?
RDP uses a lot of it's bandwidth for screen data and instruction transfer. It's not exactly designed as a file transfer protocol. I'd say 300-400kbps it probably pretty decent for what you're trying to do.
Also, if you're connecting remotely, you probably are running a VPN connection, which takes it's own connection overhead with encryption other protocol requirements.
If you need to transfer a large file between these machines, you'll probably get a much better throughput installing Dropbox on both of them and syncing files that way. http://www.dropbox.com/
Use TeamViewer - it's free and the setup is simple. Set it up on the remote using your RDP connection. I needed a 7Gbyte backup of an SQL database - RDP estimated 10 hours, and looked like it was being optimistic. I cancelled it after 30 minutes, then installed Teamviewer on the remote (about 4 minutes) Teamviewer file transfer (with the RDP connection open the whole time) - 37 minutes. Remember, if teamViewer ever asks if you need it for 'personal only', 'business' or 'both', always answer "personal only" good luck!
I just had the same trouble with RDP transfer. Also, I had trouble setting up a windows share to transfer between windows machines. So, I used the simple and well understood sftp protocol by using a free ssh/sftp server http://www.bitvise.com/ssh-server-download and an sftp/ssh client http://www.bitvise.com/ssh-client-download. I transfered 300MB(megabytes) in 2 minutes.
There is a TechNet thread that describes a similar issue to the one you described. The best answer marked by Nicholas Li from Microsoft suggests you to consider trying RichCopy. As noted, this is not considered to be a fix for the problem but you might be able to achieve better throughput.
Also, more information on Remote Desktop Protocol as well as its design and input capabilities over network connections for Windows-based applications can be found here. It may be helpful to read so as to understand the RDP architecture and features more concisely.
