Supermount removable media support

I'm getting very low transfer speeds when copying files to and from USB
devices mounted via supermount. The problem appears only for USB devices
(ie. not for CD's or DVD's), only when I mount them in read/write mode (ie.
not when mounted read-only) and only for supermount. The problem does not
appear if I mount them directly. This happens for a wide range of USB
devices: card readers, HDD's in USB racks, flash sticks, with either FAT or
ext3 filesystems on them.

Here's an example of how I use supermount:

mount -t supermount -o \
fs=vfat,dev=/dev/sda1,--,rw,uid=0,gid=100,umask=0007 \
none /mnt/usb/sda/1st

The latest kernel I've tried is vanilla 2.6.20, with supermount as the only
patch.

I'm attaching the kernel configuration. If I can help by activating
debugging on supermount please let me know what flags to use.