The discharge MOS tube itself has Cgs, Cgd, Cds parasitic capacitors, which is determined by the production process. The turning on and turning off of MOS tube is actually the process of charging and discharging Cgs. The Cgs is charged through the grid R1 resistor when it is turned on, and the charging time constant is =R1*Cgs. So if R1 is larger, the time constant is larger. In this way, if the switching frequency is very high, the tube may not be able to conduct normally within the pulse width time.