I'm guessing I'm supposed to look at propagation delay ?
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propagation delay has virtually zero impact on the debounce time


tor = RC [rc time constant] is the most significant factor by 3 orders of magnitude

falling tor = 10k*0.01uF = 100uS
propagation delay 0.021 uS

debounce time is the time taken for the st input voltage to fall from vcc to the
Negative threshold voltage for the SCHMITT trigger ie 2*tor

rising tor = 20k*0.01uF = 200uS


debounce time is the time taken for the st input voltage to rise from ground to the Positive threshold voltage for the SCHMITT trigger ie 2*tor