The problem is I don't know what kind of information I can give.
What I can say is the transfer takes 10 minutes via usb to serial instead of 30 seconds via the standard DB9 connector. It sounds like you're saying that the USB->RS232 serial connector is slower than a real RS232 port - is that right? Are you using the same program with both types of serial port? FWIW, i'm finding my USB->RS232 a poor substitute for a real RS232 port for doing some things.
