Improve the response of this circuit
If you replace the 4N25 with a logic-output optoisolator such as the TLP2363 (there are various others, some of them with second sources) you will improve the performance. Internally, they use an IC chip with a photodiode, rather than a phototransistor.
Compare the switching time of the 4N25 with your load of a few hundred ohms (from this datasheet):

Your pulse width at 5%/10kHz is 5us, so the switching time is nowhere near fast enough to get an output pulse that resembles the input pulse.

Edit: If you don't need isolation, the opto-isolator is not doing much useful.
Also, the 33 ohm resistor dissipates more than 4W when on, which could be an issue if the resistor is not capable of that much pulsed (or in a fault state, continuous) power.
Here is a similar circuit with MOSFETs. You can add a CMOS inverter in front of the circuit if you want to retain the same logic as you have now.