Given that I would look at the actual flash routines that get used and
see
if there are timing parameters that you can play with.
I checked the code, there is no timing... From what I saw, flashrom just waits for the status returned by the flash itself.
OK. Then it's possible that there's an SPI setting that says how fast the bus goes. I grepped the source for MHz and found references in it87spi.c and ichspi.c. It looks like they let you choose 20 or 33 MHz for ich and 33 or 16 MHz for it87.
It would probably help/interest more people if you said which board you are using.
Thanks, Myles