I got exactly the same Problem:
When i save a File to a Mapped Drive, it is the first Time very slow. The Second Try it works fast, but than after some Trys it slows down and the Explorer Hangs.
Reading Files is very Fast, but creating Files/Directorys is very slow.
UNC Path's don't have this Problems. This Problems occours on Samba 3.2.x and 3.5.x.
I have played around with some Samba Options, but wihtout success. I'am not sure if this is a Samba issue or Win XP.
Can you please provide a network capture that demonstrates the slowliness ?
See http://wiki.samba.org/index.php/Capture_Packets for details.