>  Why then are the SCSI drivers so slow?

Because they use polling (software repeatedly asks the card if it has data
ready to be read) rather than interrupts (card interrupts processor whenever
there's data ready to be read), etc.  Almost all SCSI and IDE drivers on 
RISC OS make use of interrupts; those on RISC OS that use polling don't
suffer because polling is fast under RISC OS, because you're always the
foreground task when doing disc transfers.  Unix does it differently.

Simon