50 kHz? So you are asking frequency, not speed. It is thermally limited basically. It depends on gate voltage (Vdd), gate resistance, ambient temperature, packaging (SMD or through hole device, maybe heat sink attached), PCB layout, and gate charge (and some other details). The total gate-drive power consumed is Pg=f*Qg*Vdd*2. It have to be dissipated by the IC, and the gate resistances. The higher the resistance the higher the portion it will dissipate. The IC internal resistance is about 10 ohms (at high temperature), so if the other gate resistances are 10 ohms too, then they will share the power equally. For example 50kHz*550nC*10V*2=0.55W, and max. allowed power dissipation is 1.6W for DIP package, so it is quite tolerable, even with 0 external gate resistance.
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