Z80s are nice processors for what they are, but considering the memory bus and instruction timing, you'd be lucky to get 6.6 MB/sec total memory throughput on a Z80 at 20 MHz, assuming completely artificial conditions (3 t-states per memory access, 20 million t-states per second). To do anything which is actually meaningful, that number would be a fraction of that (1/3 or so) because you're talking about reading one thing, writing another, and dealing with interrupts and the overhead of fetching and running instructions. An eZ80 or an embed-intended MIPS or StrongARM would be much better suited.

Then again, 1 MB/sec on many m68k machines isn't too shabby. It wouldn't compare with a decent SCSI drive on the motherboard SCSI, but it wouldn't feel THAT much slower... But compared with wide and/or Ultra SCSI, it wouldn't come close.

NuBus on earlier Macs was stuck at 10 MHz and was updated to 20 MHz on the AV machines and PowerPC machines, so the maximum speed was 20 MB/sec, although speeds between 10 and 20 are more reasonable to expect. From what I remember, all Quadras support the NuBus90 standard, but can only talk at 20 MHz between two cards; everything to and from the system was at 10 MHz.