Hi Rich
Well, Mach starts by letting the spindle spin a few times to get a nice average spindle speed - that is true. So having established that the spindle is running at 300 rpm, it starts the Z axis travelling in sync. Some mm later the tool hits the metal - and the spindle drops to 360 rpm. Mach will not know anything about that until the next index pulse arrives. So it HAS to assume that the the spindle speed has stayed constant, until it gets evidence that it has not.
To be sure, if you have a 3 kW synchronous AC motor driving the spindle through belt reduction, and the thread is M6x1, the motor will not be affected by the load. But if you have a hobbyist machine with a 500 W (DC or AC) motor, as so many do, things are different. And so we get repeated complaints that 'threading is not working'.
Cheers