Try removing the SB card and see if that makes any difference. I have a PCMCIA controller card and a 2GB PCMCIA Toshiba disk drive (not flash) and transfers to it are very slow. I've never resolved the problem, but remember reading about a poorly written SB driver that polls the processor (via the PCI bus) and it was these hardware interrupts that caused poor performance.

Although my confidence is low that this is your problem, it's any easy thing to try removing the sound card. Good luck.

-Zeke