slew rate of a mosfet
A rough approximate is to use the RdsOn for -5V.
It is guaranteed to be 0.3Ω but that is for -10V @25'C with 7.2A Pulse, width = 300 μs; duty cycle = 2 %. 
your situation is different and vague; 


Initial conditions : 
Vcap (unknown )
Cap part number (unknown ) and 
trace inductance (unknown ) 
cap ESR or Dissipation Factor (unknown )
Diode ESR 1N4148  0.1W ~ 10ohm ( assumed to be larger than 1nF     
Cap ESR which is expected to have ESR*C value=T &lt; 0.1us for ceramic
The RdsOn starts at a high value due to Vdes =5V and Vgs=-5V and not being a "logic level" gate controlled FET requires a certain amount of calculations from the datasheet below




With the slew rate being dV/dt=Ic/C for the cap and Ic = (V+-Vcap)/(RdsOn+ESR(diode)) it becomes highly nonlinear.

But with initial conditions of ESR diode=10, and ESR or RdsOn of FET=10Ω then dropping to 0.34 Ω as Vds drops below 3V.


Ic=C dV/dt=1e-9 * (5V)/(10Ω+10Ω) and C= 1e-9F for Vds=3~5V


slew rate dV/dt= Ic/C= 5V/20Ω*1e9= = 0.25V/ns

Low confidence in results due to inadequate info.