People have run Hitachi 68C09Es at 5 MHz. May not sound like much, but the 6809 (which the 6309 is an improved version of) takes fewer clock cycles than Z-80 or 808x take to do similar work. OpenCores has a 6809 core, not cycle accurate, that can run at 40 MHz if I understand the description rightly (https://opencores.org/projects/6809_6309_compatible_core).
