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what makes the steps time sensitive?
They give the stepper motors speed. Above some base speed, you can't change speed deliberately, not even on stepper motors. You have to accelerate and decelerate properly. Accordingly, gaps in the step pin timing lead to step losses.
That said, shift registers can be very quick. You can fill them at 10 MHz, or all 8 bits in less than one microsecond. Step rates are in the 20 ... 100 microsecond range at top speed, so there's some allowance.