The spindle encoder runs the feed during threading.
With normal turning the Z-Axis is clocked based on distance per minute.
When threading the Z-axis is clocked based on pulses per revolution
When threading the clock that moves the Z-axis is derived from the spindle encoder.
When turning a thread the tool is often outside the the job before the thread starts.
The Z-axis cannot instantaneously accelerate up to the speed required by the clock, so in turning a thread if the tool is engaged in the job at the start of the thread you would see a thread changing in pitch.
This thread changing in pitch is normally out in mid space so you never see it.
At the finish end of the thread the Z-axis must stop and/or retract.
It is usually retracted so so you don't see a change in pitch. The Z-axis cannot STOP instantly either.
The faster you run the spindle, the more severe is the acceleration/stopping required by the Z-axis. At slow spindle speed, the effect will be quite small.
As the speed increases, this pitch change error can become very apparent.
That will be what limits the speed, (ignoring surface speed) cutting a thread.
Now back to your question. All of the above applies
PLUS
what effect will the changing pitch have on the tap at the start?
Going into the hole it can be accelerated in mid space. - No problem.
The faster the spindle speed the more room needed to accelerate.
When it is time to reverse, the spindle comes to a stop (and I bet the Z-axis has no problem keeping with that). It now reverses and clocks back out of the hole.
You just need to allow for the time it takes the spindle to reverse, so as the speed increases, the possibility of overshooting and trying to tap too deep becomes the problem.
If the machine controller is smart enough it will reverse the spindle at exactly the correct Z-depth and take care of all this for you.
As long as it can stop properly before reversing is the limiting factor.
If you are spinning to fast and it takes 15 turns to stop and you only have
have 10 threads to feed you are in trouble.
Does the user programming manual have some good examples?
Super X3. 3600rpm. Sheridan 6"x24" Lathe + more. Three ways to fix things: The right way, the other way, and maybe your way, which is possibly a faster wrong way.