Just been thinking about your circuit. Can your driving source drive into a 56R with a LED in series with it (I ~50mA). Also the FET does not require much power only voltage, so I think the IR2110 is overkill. What sort of frequency is your PWM? If you preceed your FET gate with an emitter follower, then with its base returned to the 0V line via a 10K, and the photodiode strung between its base base and +Vcc with a 1K series (current limiting) resistor. Then depending on the current transfer ratio of the opto isolator, 20mA into LED = > 10mA output into base of emitter follower, the emitter follower will then be capable of outputing 10 mA X Hfe > 500mA? This will pull the gate positive quickly, but the discharge will be via the emitter resistor so this should be as low as possible consistent with getting the change of voltage on the FET gate ( 1k to start with?).
Frank