Anything around 1 second is not cable capacitance. Cablle capacitance is in picofarads... The RC time constant is simply R x C, so that 1uF into 1Meg Ohm is one second, or 1000uF into 100 Ohms.
LDRs are known for being slow, but I don't think they are that slow and I think there is something else going on.
How fast are the pulses?