The steps per direct relate to pulses so if you exceed 25,000 you exceed the default kernel speed.
See the attached .jpg
Choosing Kernel Speed
The Mach3 driver can run at frequencies from 25,000 Hz (pulses per second) up to 100,000 Hz,
depending on the speed of your processor and other loads placed on it when running Mach3.
The frequency you need depends on the maximum pulse rate you need to drive any axis at its top
speed. 25,000 Hz will probably be suitable for stepper motor systems. With a 10 micro-step driver
such as a Gecko 201, you will get around 750 RPM from a standard 1.8 degree stepper motor with a
25,000 Hz pulse rate. Higher pulse rates are needed to achieve desired motor RPM for servo drives that
have high resolution shaft encoders on the motor. Further details are given in the section on motor tuning
and in Section 4.4.2, Determining Axis Drive Requirements.
Computers with a 1 GHz clock speed will almost certainly be able to run at 35,000 Hz, so you can
choose this if you need a high step rate (for example, if you have very fine pitch lead screws).
The demonstration version of Mach3 will run at 25,000 Hz only. In addition, if Mach3 is forcibly
closed, then on re-start it will automatically revert to 25,000 Hz operation. The actual frequency of the
running system is displayed on the standard Diagnostics screen.
Click the box next to the desired kernel speed.
Don't forget to click the Apply button before proceeding.