It's not that the Z80 "requires a 4x clock", but four T-states (clock cycles) make up one machine cycle. Instructions take anywhere from one to several machine cycles to execute, depending on the instruction.
This is pretty much my recollection. My Zeta system I knocked up recently as part of the N8VEM project can have usually 6-20Mhz oscillators assuming you have correct divider crystal for the disk controller (FCD266). At 20Mhz, the thing goes like stink compared to my spectrum. Decompresing 128kb via UNARC.COM is perhaps 6-7 seconds. Spectrum+3 doing the same deal is around 40 seconds and the C128Dcr rumbles in at around 2min 10s because the Z80 is running effectively at 2Mhz becuase it shares the bus with the MOS6502 derivative. Old communist UB880-D (Z80 clone) at 2mhz takes ~4 minutes (I wandered off to make a cuppa).
I was talking more "higher level". Have you guys checked out the Zilog ez80's? They don't support the full z80 ISA but that can be micro-coded/patched pretty simply. I've seen one project that ran a port of CP/M 2.2C and ZSYS. I don't care how much emulation of a couple of missing instructions it has to do, but they run at 50Mhz and have a full
4x4x4 pipeline. That'd be 'quick' in my books. Al. -- -- Al Boyanich adb -w -P "world> " -k /dev/meta/galaxy/ksyms /dev/god/brain