your valve has minimum speed requirement (near dc control signal)
not sure if i understand this?? were you thinking that the control circuity in the valve needs to be updated at a certain minium frequency? which would mean the settling time of the RC portion needs to be somewhat fast?
how much would things change if i would change the PWM frequency that the Arduino is pushing out?
how is the pull-up resistor affecting the R/C filter, or does it? ( i would think this might be 'magic')
..and how did you come up with charge / discharge time ??