The effect largely goes away if the frequency exceeds 4 kHz, as even the quickest saccades would see a blur rather than a flicker. putting the flicker above 20 kHz would be ideal, because that way it is well beyond both visual range and even any noise generated by the circuit would be inaudible. The real question is why higher frequencies aren't used for driving LEDs. They have very low cycling latency, so it's a no-brainer.