I think that was the whole point of having both CPUs on the Phase5 boards, because they could not figure out a way to emulate the 68k series chips fast enough, so they ended up with the kludge of switching back and forth between the PPC and the 68k CPUs.
OS4 for Classic should give you an idea of how fast the native 68k emulation is, as I believe that it does not use the actual 68k CPU on the CyberStorm & Blizzard cards.
