I believe that the software is looking at the DSP clock on the CPU7/CPU10 board for its time reference, and so is not dependent on the speed of the PC processor. Not that it would be anyway; there are plenty of reliable ways to get reasonably accurate times on the PC side which are independent of CPU speed.

The DSP clock on the CPU7/CPU10 should update at 4kHz (the chip's interrupt rate). That counter is made visible to the PC software through a dual-port RAM (memory-mapped I/O). If it were not updating at 4kHz, you would have serious axis-control problems, because it is that 4kHz interrupt which runs the PID position-control loop.

Also, if the clock were counting too fast, then the reported spindle speed would be low (fewer encoder counts seen per each interrupt). In order for the DSP clock count to cause the spindle speed to read high, it would have to count too slow.

Did you do anything to change the clock crystal associated with the ADSP-2101 chip on your Centroid CPU board?