It is all in the numbers.
Figure out the RPM you want to run the motors and the numbers of steps required to achive that , for example (using round numbers);
Steppers 1,000 RPM x 200 steps per rev = 200,000 pulses (per minute) which is only about 3,300 pulses per second, so 25,000 kernel speed would be plenty, however, if you have 10 micro step drive, suddenly you need 33,000 pulses.
Servo 4,000 RPM x 8000 steps per rev = 32,000,000 pulses per minute which is 53,300 steps per second, so you yound need to run 60k kernel speed to get full rpm.
Alternatives to full kernel speed in Mach are pulse mulripliers in the drives or a pulse gerenator independent to Mach3 such as Smoothstepper or Kflop.
A few years ago is a long time in computer history. I just put together what I would call a 'budget' build using 2 gerations out-of-date technology (socket 775) and it is running Mach at 100k (so far). A good test of headroom is to open the task manager and set it on performance and then drag it rapidly around the screen as Mach is running a program.