There is absolutely *nothing* to be gained by running the Mach3 kernel speed any faster than the fastest step rate you require. All it does it eat up CPU bandwidth, and potentially create problems. Few machines seem capable of running reliably long-term above a 45kHz kernel rate, regardless of what the driver test indicates. If it works at 35kHz, and gives you the rapid speeds you need, then run it at 35kHz. Going faster buys you nothing at all.
Regards,
Ray L.