IRF520 max frequency
Nano output can only switch at 8 MHz max (except for the system clock), so the IFR520 could only change state every 125nS max. (62.5nS x 2 = 125nS, so one rising edge at time 0, followed by a falling edge 62.5nS later, then a rising edge at 125nS, and so on).

You should also look at logic level MOSFET, as th IRF520 is barely turned on with a logic level on the gate (Vgs(th) (Max) @ : 4V @ 250µA), and needs 10V to turn fully on; where as a logic level part such as AOD510/514/518 will turn fully on to conduct with a very low Rds (0.004 ohm for AOD510) at that level. (AOD514 and AOD518 are a few milliohm higher, check datasheets at Digikey.)