How do I speed up transistor switching in low side power switch?
While you do not make clear what the voltage of the PWM is, The voltage on R21 should be half the supply voltage. this will remove the influence of any capacitance at the inputs from slowing down the transition, 

Next up, would be loose R32, the whole point of U4 is to slam that mosfet on and off as fast as possible, that resistor is preventing it, with a normal op amp it would be recommended, but this device is intended to deal with capacitive loads.