IEEE 802.11 specifies a standard slot time of 20 μsec and 9 μsec for short slots in 802.11g. You can manipulate the slot time by writing the time value (20 or 9) to SHM location 0x10. At the same time, MMIO offset 0x684 should be updated to 510 + the desired slot time. I'm open for suggestions as to what 510 + slot time means.
Note that some initial values set the slot time as well.