The thing is that if the pulse is quite short, you run the risk of the input not being detected, the shorter the pulse the lower the probability of it being detected.
For a 100% detection, around 1 seconds of hold is required. If it so happens, that on a short pulse it was detected, great.
The low detection rate for slow pulses is related to the device hardware, in very regular pulses, you could detect maybe 4 pulses a seconds, with 50% duty cycle, meaning the pulse is on 125ms and pulse is of for 125ms.
Maybe RUT956 RUT951 be better at this because of the new chipset, I am not sure, we have not tested that