ZX-24a reading RPM information . . .

[dkinzer]

Should i also change the ShiftOut to the 74595 to bit banging ?

Note that you can use ShiftOutEx() to shift out up to 16 bits in one call. Also, ShiftOutEx() has more options for speed control, including the ability to shift at high speed (~2.2uS per bit). For 16-bit data, this would take about 35uS while 8-bit data would take about 17uS. If these interrupt disable times are too large, you can implement a bit-banged shift out function that would take longer overall but would not have interrupts disabled for any significant time.