Operational Ampifier integrator dead time in proteus 8

Hi! I'm using Proteus 8 to simulate a sample and hold circuit that involves an integrator op amp that gets from 4v to 0v in 400us when the input goes down to 0v; but the slope beginning seems to have a delay of about 150us.
I just want to know if this will happen in real life (therefore, I'd have to compensate for the delay) or it s some sort of a bug. I can't figure out why does this happen either.

Circuit simulation info:
op amp: LM324, spice model LM324/NS from the NATOA library.
C = 0.01uF, CAP from the DEVICE library.
R = 20k, RES from the DEVICE library.

this circuit is single supply:
+Vcc = +5v;
-Vcc = gnd;
Vin+ = 2v