Single axis cnc controller

You should really post your complete sketch.

This sounds like you were using Serial.readString() which has a too long timeout.

This is unclear to me. I do not yet understand what you want to say with that.
What is a "time-buffer" ? you can't buffer time !
So this means you are buffering something else.
What exactly are you buffering?

coordinates?
steps to move?

Why does it take 750 milliseconds ?
Did you measure that it takes 750 milliseconds between
....

yeah !? between exactly what?
sending coordinates and reaction of the stepper-motors?

How does the code look like that you use to drive the stepper-motors?
==> post your complete sketch !
using the method like described here

best regards Stefan