What is the upper frequency of a zener used for transient voltage suppression?
Diode capacitance likely limits the time response of this circuit. Avalanche diode capacitance depends on its reverse-bias voltage, and is maximum for low voltage, while minimum as you approach the reverse-breakdown voltage. It acts very much like a varactor diode (and can be used as a voltage-variable capacitor).
Since the 12V_INPUT is some kind of a logical signal, the diode is swept from a low voltage up to near its breakdown limit, so its capacitance changes over a wide range, perhaps from 100 pf to 10 pf.
Taking worst case (100 pf), a 500 kHz input signal would be very close to the upper limit, and would cause some edge distortion. This might be a good thing, offering some minor noise immunity.